I made a plugin and tested it on b2evo 2.4.5. You can see an example here http://blogs.lessthandot.com/index.php/All/?disp=authdir

Use and improve this as you like.

Here is the first file the biggest too.

I called it _authors.plugin.php and it goes in the plugins folder.

<?php
/**
 * This file implements the Authors plugin. This plugin is based on the Archives plugin.
 *
 * Displays a list of posts per author.
 *
 * @package plugins
 *
 * {@internal Below is a list of authors who have contributed to design/coding of this file: }}
 * @author chrissie1: Christiaan Baes.
 *
 * @version $Id: _authors.plugin.php,v 0.1 2009/01/03 $
 */
if( !defined('EVO_MAIN_INIT') ) die( 'Please, do not access this page directly.' );
 
 
load_funcs('_core/ui/results/_results.class.php');
 
 
 
/**
 * Authors Plugin
 *
 * This plugin displays
 */
class authors_plugin extends Plugin
{
    /**
     * Variables below MUST be overriden by plugin implementations,
     * either in the subclass declaration or in the subclass constructor.
     */
 
    var $name = 'Authors Widget';
    var $code = 'evo_Auth';
    var $priority = 50;
    var $version = '0.1';
    var $author = 'chrissie1';
    var $group = 'widget';
 
 
    /**
     * Init
     */
    function PluginInit( & $params )
    {
        $this->short_desc = T_('This skin tag displays a list of posts per author.');
        $this->long_desc = T_('Post are grouped by author.');
 
        $this->dbtable = 'T_items__item';
        $this->dbprefix = 'post_';
        $this->dbIDname = 'post_ID';
    }
 
 
    /**
     * Event handler: SkinTag
     *
     * @param array Associative array of parameters. Valid keys are:
     *                - 'block_start' : (Default: '<div class="bSideItem">')
     *                - 'block_end' : (Default: '</div>')
     *                - 'title' : (Default: T_('Authors'))
     *                - 'mode' : 'list|author' 
     *                - 'author' : 
     *                - 'limit' : # of archive entries to display or '' (Default: 12)
     *                - 'more_link' : more link text (Default: 'More...')
     *                - 'list_start' : (Default '<ul>')
     *                - 'list_end' : (Default '</ul>')
     *                - 'line_start' : (Default '<li>')
     *                - 'line_end' : (Default '</li>')
     *                - 'day_date_format' : (Default: conf.)
     * @return boolean did we display?
     */
    function SkinTag( $params )
    {
        /**
         * @var Blog
         */
        global $Blog;
 
        if( empty($Blog) )
        {
            return false;
        }
 
        /**
         * Default params:
         */
        // This is what will enclose the block in the skin:
        if(!isset($params['block_start'])) $params['block_start'] = '<div class="bSideItem">';
        if(!isset($params['block_end'])) $params['block_end'] = "</div>\n";
 
        // Title:
        if(!isset($params['block_title_start'])) $params['block_title_start'] = '<h3>';
        if(!isset($params['block_title_end'])) $params['block_title_end'] = '</h3>';
        // Authors mode:
        if(!isset($params['mode']))
            $params['mode'] = 'list';
        if($params['mode']=='author')
        {
            if(!isset($params['title'])) $params['title'] = T_('Author');
        }
        else
        {
            if(!isset($params['title'])) $params['title'] = T_('Authors');
        }
        
        // Number of archive entries to display:
        if(!isset($params['limit'])) $params['limit'] = 12;
        
        // Number of archive entries to display:
        if(!isset($params['author'])) $params['author'] = '';
 
        // More link text:
        if(!isset($params['more_link'])) $params['more_link'] = T_('More...');
 
        // This is what will enclose the list:
        if(!isset($params['list_start'])) $params['list_start'] = '<ul>';
        if(!isset($params['list_end'])) $params['list_end'] = "</ul>\n";
 
        // This is what will separate the archive links:
        if(!isset($params['line_start'])) $params['line_start'] = '<li>';
        if(!isset($params['line_end'])) $params['line_end'] = "</li>\n";
 
        $AuthorsList = & new AuthorsList( $params['mode'], $params['limit'], $params['author'], $this->dbtable, $this->dbprefix, $this->dbIDname );
 
        echo $params['block_start'];
 
        if( !empty($params['title']) )
        {   // We want to display a title for the widget block:
            echo $params['block_title_start'];
            echo $params['title'];
            echo $params['block_title_end'];
        }
 
        if ($AuthorsList->total_rows>0)
        {
            echo $params['list_start'];
            if($params['mode']=='author')
            {
                $AuthorsList->get_item( $arc_author_ID, $arc_author, $arc_count, $post_ID, $post_title, $views, $comments);
                echo '<h2>'.$arc_author.'</H2>';
                echo '<br />';
                echo $params['line_start'];
                if($post_title=='')
                {
                    $post_title='No title';
                }
                echo '<a href="'.url_add_param( $Blog->get('url'), 'p='.$post_ID ).'">'.$post_title.'</a> (Views: '.$views.', Comments: '.$comments.')';
                echo $params['line_end'];
            }
            while( $AuthorsList->get_item( $arc_author_ID, $arc_author, $arc_count, $post_ID, $post_title, $views, $comments) )
            {
                echo $params['line_start'];
                switch( $params['mode'] )
                {
                    case 'list':
                        echo '<a href="'.url_add_param( $Blog->get('url'), 'disp=authdir&amp;author='.$arc_author_ID ).'">'.$arc_author.'</a>';
                        echo ' <span class="dimmed">('.$arc_count.')</span>';
                        break;
 
                    case 'author':
                        if($post_title=='')
                        {
                            $post_title='No title';
                        }
                        echo '<a href="'.url_add_param( $Blog->get('url'), 'p='.$post_ID ).'">'.$post_title.'</a> (Views: '.$views.', Comments: '.$comments.')';
                        break;
                }
 
                echo $params['line_end'];
            }
 
            // Display more link:
            if( !empty($params['more_link']) )
            {
                echo $params['line_start'];
                echo '<a href="'.url_add_param( $Blog->get('url'), 'disp=authdir' );
                echo '">'.format_to_output($params['more_link']).'</a>';
                echo $params['line_end'];
            }
        
        
            echo $params['list_end'];
        }
        else
        {
            echo 'Author not found.';
        }
        
        if($params['mode']=='author')
        {
            echo '<br />';
            echo '<br />';
            echo '<a href="'.url_add_param( $Blog->get('url'), 'disp=authdir' );
            echo '">Back to authorslist</a>';
        }
 
        echo $params['block_end'];
 
        return true;
    }
}
 
 
/**
 * Archive List Class
 *
 * @package evocore
 */
class AuthorsList extends Results
{
    var $archive_mode;
    var $author;
 
    /**
     * Constructor
     *
     * Note: Weekly archives use MySQL's week numbering and MySQL default if applicable.
     * In MySQL < 4.0.14, WEEK() always uses mode 0: Week starts on Sunday;
     * Value range is 0 to 53; week 1 is the first week that starts in this year.
     *
     * @link http://dev.mysql.com/doc/mysql/en/date-and-time-functions.html
     *
     * @todo categories combined with 'ALL' are not supported (will output too many archives,
     * some of which will resolve to no results). We need subqueries to support this efficiently.
     *
     * @param string
     * @param integer
     * @param boolean
     */
    function AuthorsList(
        $archive_mode = 'list',
        $limit = 100,
        $author = '',
        $dbtable = 'T_items__item',
        $dbprefix = 'post_',
        $dbIDname = 'ID' )
    {
        global $DB;
        global $blog, $cat, $catsel;
        global $show_statuses;
        global $timestamp_min, $timestamp_max;
        global $s, $sentence, $exact;
 
        $this->dbtable = $dbtable;
        $this->dbprefix = $dbprefix;
        $this->dbIDname = $dbIDname;
        $this->archive_mode = $archive_mode;
        $this->author = $author;
 
        /*
         * WE ARE GOING TO CONSTRUCT THE WHERE CLAUSE...
         */
        $this->ItemQuery = & new ItemQuery( $this->dbtable, $this->dbprefix, $this->dbIDname ); // TEMPORARY OBJ
 
        $this->ItemQuery->where_chapter( $blog, '', array() );
 
        // * Restrict to the statuses we want to show:
        $this->ItemQuery->where_visibility( $show_statuses );
 
        // - - - + * * timestamp restrictions:
        $this->ItemQuery->where_datestart( '', '', '', '', $timestamp_min, $timestamp_max );
 
 
        $this->from = $this->ItemQuery->get_from();
        $this->where = $this->ItemQuery->get_where();
        $this->group_by = $this->ItemQuery->get_group_by();
 
        switch( $this->archive_mode )
        {
            case 'list':
                $sql = 'SELECT                      post_creator_user_ID,
                                                    user_login,
                                                    COUNT(DISTINCT postcat_post_ID) AS count '
                                                    .$this->from . ', evo_users '
                                                    .$this->where.'
                                                    AND post_creator_user_ID = user_ID
                                                    GROUP BY post_creator_user_ID
                                                    ORDER BY COUNT(DISTINCT postcat_post_ID) DESC';
                break;
 
            case 'author':
                $sql = "SELECT                      DISTINCT '.$this->dbIDname.' 
                                                    ,post_ID
                                                    ,user_login
                                                    ,post_title
                                                    ,post_views 
                                                    ,(select count(*) from evo_comments where comment_post_ID = post_id and comment_status = 'published') as comments "
                                                    .$this->from. ", evo_users "
                                                    .$this->where."
                                                    AND post_creator_user_ID = ".$author."
                                                    AND post_creator_user_ID = user_ID
                                                    GROUP BY post_ID
                                                    ORDER BY post_datestart DESC";
                break;
 
        }
 
        parent::Results( $sql, 'AuthorsList_', '', $limit );
 
        $this->restart();
    }
 
 
    /**
     * Count the number of rows of the SQL result
     *
     * These queries are complex enough for us not to have to rewrite them:
     * dh> ???
     */
    function count_total_rows()
    {
        global $DB;
 
        switch( $this->archive_mode )
        {
            case 'list':
                $sql_count = 'SELECT COUNT( DISTINCT  post_creator_user_ID) '
                                                    .$this->from
                                                    .$this->where;
                break;
 
            case 'author':
                $sql_count = 'SELECT COUNT( DISTINCT post_ID)'
                                                    .$this->from
                                                    .$this->where.'
                                                    AND post_creator_user_ID = '.$this->author;
                break;
 
        }
 
        $this->total_rows = $DB->get_var( $sql_count ); //count total rows
    }
 
 
    /**
     * Rewind resultset
     */
    function restart()
    {
        // Make sure query has executed at least once:
        $this->query();
 
        $this->current_idx = 0;
    }
 
    /**
     * Getting next item in archive list
     *
     * WARNING: these are *NOT* Item objects!
     */
    function get_item( & $arc_author_ID, & $arc_author, & $arc_count, & $post_ID, & $post_title, & $views, & $comments )
    {
        if( $this->current_idx >= $this->result_num_rows )
        {   // No more entry
            return false;
        }
 
        $arc_row = $this->rows[ $this->current_idx++ ];
 
        switch( $this->archive_mode )
        {
            case 'list':
                $arc_author_ID  = $arc_row->post_creator_user_ID;
                $arc_author  = $arc_row->user_login;
                $arc_count = $arc_row->count;
                return true;
 
            case 'author':
                $post_ID = $arc_row->post_ID;
                $arc_author  = $arc_row->user_login;
                $post_title = $arc_row->{$this->dbprefix.'title'};
                $views = $arc_row->post_views;
                $comments = $arc_row->comments;
                return true;
        }
    }
}
 
 
 
 
 
?>

Then I created this file. _authdir.disp.php and that goes in the skins folder.

<?php
/**
 * This is the template that displays the archive directory for a blog
 *
 * This file is not meant to be called directly.
 * It is meant to be called by an include in the main.page.php template.
 * To display the archive directory, you should call a stub AND pass the right parameters
 * For example: /blogs/index.php?disp=authdir
 *
 * b2evolution - {@link http://b2evolution.net/}
 * Released under GNU GPL License - {@link http://b2evolution.net/about/license.html}
 * @copyright (c)2003-2007 by Francois PLANQUE - {@link http://fplanque.net/}
 *
 * @package evoskins
 */
if( !defined('EVO_MAIN_INIT') ) die( 'Please, do not access this page directly.' );
 
$author = param('author');
if(!is_numeric($author))
{$author='';}
if (!$author=='')
{
    // Call the Authors plugin WITH NO LIMIT & NO MORE LINK:
    $Plugins->call_by_code( 'evo_Auth', array( 'mode'=>'author',
                                                'author'=>$author,
                                              'block_start'=>'',
                                              'block_end'=>'',
                                              'limit'=>'',
                                              'more_link'=>'' ) );
}
else
{
    // Call the Authors plugin WITH NO LIMIT & NO MORE LINK:
    $Plugins->call_by_code( 'evo_Auth', array( 'title'=>'',
                                              'block_start'=>'',
                                              'block_end'=>'',
                                              'limit'=>'',
                                              'more_link'=>'' ) );
}
?>

I also had to adapt the _skin.func.php file a little in the inc/skins folder.

I added this line of code

'disp_authdir'         => '_authdir.disp.php',

after this line of code.

'disp_arcdir'         => '_arcdir.disp.php',