Far more secure for you as admin to "upgrade" a user to the correct group than to allow any man and his dog to choose his access level on (open) registration ;)

¥

That would be right, but the groups will have restricted permissions, so the new users won't choose their access levels, because I would administrate the group permission.

e.g. I have 3 blogs and every blog has its own group (only these group memebers can write post in this special blog).
So, the new user can decide for which blog he/she wants to register.

Moved do feature requests

We can make some groups "open" for registration, so users may choose from those "open" groups. I think this makes sense.

That would be great, how and where can I do this??

You can't do this, it's the note/request for devs. We can just wait for replies and hope that they will add this in next release.

BTW this can be easily done with plugin.

And does such a plugin exits? Or do I have to develop it on my own?
thx

You'd need to write your own plugin.

Take a look at /inc/plugins/model/_plugins.admin.class.php ( approx line 52+ ) for all teh supported plugin events :

	/**
	 * Get the list of all events/hooks supported by the plugin framework.
	 *
	 * Also puts in additional events provided by plugins.
	 * fp> please provide an example/use case
	 *
	 * Additional to the returned event methods (which can be disabled), there are internal
	 * ones which just get called on the plugin (and get not remembered in T_pluginevents), e.g.:
	 *  - AfterInstall
	 *  - BeforeEnable
	 *  - BeforeDisable
	 *  - BeforeInstall
	 *  - BeforeUninstall
	 *  - BeforeUninstallPayload
	 *  - DisplaySkin (called on a skin from {@link GetProvidedSkins()})
	 *  - ExecCronJob
	 *  - GetDefaultSettings
	 *  - GetDefaultUserSettings
	 *  - GetExtraEvents
	 *  - GetHtsrvMethods
	 *  - PluginInit
	 *  - PluginSettingsUpdateAction (Called as action before updating the plugin's settings)
	 *  - PluginSettingsEditAction (Called as action before editing the plugin's settings)
	 *  - PluginSettingsEditDisplayAfter (Called after standard plugin settings are displayed for editing)
	 *  - PluginSettingsValidateSet (Called before setting a plugin's setting in the backoffice)
	 *  - PluginUserSettingsUpdateAction (Called as action before updating the plugin's user settings)
	 *  - PluginUserSettingsEditDisplayAfter (Called after displaying normal user settings)
	 *  - PluginUserSettingsValidateSet (Called before setting a plugin's user setting in the backoffice)
	 *  - PluginVersionChanged (Called when we detect a version change)
	 *
	 *  The max length of event names is 40 chars (T_pluginevents.pevt_event).
	 *
	 * @todo Finish/Complete descriptions
	 *
	 * @return array Name of event (key) => description (value)
	 */
	function get_supported_events()
	{
		static $supported_events;

		if( empty( $supported_events ) )
		{
			$supported_events = array(
				'AdminAfterPageFooter' => '',
				'AdminDisplayEditorButton' => '',
				'AdminDisplayToolbar' => 'Display a toolbar on the edit screen(s)',
				'AdminDisplayCommentFormFieldset' => '',
				'AdminDisplayItemFormFieldset' => '',
				'AdminEndHtmlHead' => '',
				'AdminAfterMenuInit' => '',
				'AdminTabAction' => '',
				'AdminTabPayload' => '',
				'AdminToolAction' => '',
				'AdminToolPayload' => '',

				'AdminBeforeItemEditCreate' => 'This gets called before a new item gets created from the backoffice.',
				'AdminBeforeItemEditUpdate' => 'This gets called before an existing item gets updated from the backoffice.',
				'AdminBeforeItemEditDelete' => 'This gets called before an existing item gets deleted from the backoffice.',

				'AdminBeginPayload' => '',

				'CacheObjects' => 'Cache data objects.',
				'CachePageContent' => 'Cache page content.',
				'CacheIsCollectingContent' => 'Gets asked for if we are generating cached content.',

				'AfterCommentDelete' => 'Gets called after a comment has been deleted from the database.',
				'AfterCommentInsert' => 'Gets called after a comment has been inserted into the database.',
				'AfterCommentUpdate' => 'Gets called after a comment has been updated in the database.',

				'AfterItemDelete' => '',
				'PrependItemInsertTransact' => '',
				'AfterItemInsert' => '',
				'PrependItemUpdateTransact' => '',
				'AfterItemUpdate' => '',
				'AppendItemPreviewTransact' => '',

 				'FilterItemContents' => 'Filters the content of a post/item right after input.',
 				'UnfilterItemContents' => 'Unfilters the content of a post/item right before editing.',

 				// fp> rename to "PreRender"
				'RenderItemAsHtml' => 'Renders content when generated as HTML.',
				'RenderItemAsXml' => 'Renders content when generated as XML.',
				'RenderItemAsText' => 'Renders content when generated as plain text.',

				// fp> rename to "DispRender"
				// dh> TODO: those do not get called anymore!
				'DisplayItemAsHtml' => 'Called on an item when it gets displayed as HTML.',
				'DisplayItemAsXml' => 'Called on an item when it gets displayed as XML.',
				'DisplayItemAsText' => 'Called on an item when it gets displayed as text.',

// fp> These is actually RENDERing, right?
// TODO: Rename to "DispRender"
				'FilterCommentAuthor' => 'Filters the comment author.',
				'FilterCommentAuthorUrl' => 'Filters the URL of the comment author.',
				'FilterCommentContent' => 'Filters the content of a comment.',

				'AfterUserDelete' => '',
				'AfterUserInsert' => '',
				'AfterUserUpdate' => '',

// fp> This is actually RENDERing, right?
// TODO: Rename to "DispRender"
				'FilterIpAddress' => 'Called when displaying an IP address.',

				'ItemApplyAsRenderer' => 'Asks the plugin if it wants to apply as a renderer for an item.',
				'ItemCanComment' => 'Asks the plugin if an item can receive comments/feedback.',
				'ItemSendPing' => 'Send a ping to a service about new items.',
				'ItemViewsIncreased' => 'Called when the view counter of an item got increased.',

				'SkinTag' => '',

				'AppendHitLog' => 'Called when a hit gets logged, but before it gets recorded.',

				'DisplayCommentToolbar' => 'Display a toolbar on the public feedback form',
				'DisplayCommentFormButton' => '',
				'DisplayCommentFormFieldset' => '',
				'DisplayMessageFormButton' => '',
				'DisplayMessageFormFieldset' => '',
				'DisplayLoginFormFieldset' => 'Called when displaying the "Login" form.',
				'DisplayRegisterFormFieldset' => 'Called when displaying the "Register" form.',
				'DisplayValidateAccountFormFieldset' => 'Called when displaying the "Validate account" form.',

				'CommentFormSent' => 'Called when a public comment form has been sent and gets received.',
				'BeforeCommentFormInsert' => 'Called before a comment gets recorded through the public comment form.',
				'AfterCommentFormInsert' => 'Called after a comment has been added through public form.',

				'BeforeTrackbackInsert' => 'Gets called before a trackback gets recorded.',
				'AfterTrackbackInsert' => 'Gets called after a trackback has been recorded.',

				'LoginAttempt' => 'Called when a user tries to login.',
				'LoginAttemptNeedsRawPassword' => 'A plugin has to return true here, if it needs a raw (un-hashed) password in LoginAttempt.',
				'AlternateAuthentication' => '',
				'MessageFormSent' => 'Called when the "Message to user" form has been submitted.',
				'MessageFormSentCleanup' => 'Called after a email message has been sent through public form.',
				'Logout' => 'Called when a user logs out.',

				'GetSpamKarmaForComment' => 'Asks plugin for the spam karma of a comment/trackback.',

				// Other Plugins can use this:
				'CaptchaValidated' => 'Validate the test from CaptchaPayload to detect humans.',
				'CaptchaValidatedCleanup' => 'Cleanup data used for CaptchaValidated.',
				'CaptchaPayload' => 'Provide a turing test to detect humans.',

				'RegisterFormSent' => 'Called when the "Register" form has been submitted.',
				'ValidateAccountFormSent' => 'Called when the "Validate account" form has been submitted.',
				'AppendUserRegistrTransact' => 'Gets appended to the transaction that creates a new user on registration.',
				'AfterUserRegistration' => 'Gets called after a new user has registered.',

				'SessionLoaded' => '', // gets called after $Session is initialized, quite early.

				'AfterLoginAnonymousUser' => 'Gets called at the end of the login procedure for anonymous visitors.',
				'AfterLoginRegisteredUser' => 'Gets called at the end of the login procedure for registered users.',

				'BeforeBlogDisplay' => 'Gets called before a (part of the blog) gets displayed.',
				'SkinBeginHtmlHead' => 'Gets called at the top of the HTML HEAD section in a skin.',
				'SkinEndHtmlBody' => 'Gets called at the end of the skin\'s HTML BODY section.',
				'DisplayTrackbackAddr' => '',

				'GetCronJobs' => 'Gets a list of implemented cron jobs.',
				'GetProvidedSkins' => 'Get a list of "skins" handled by the plugin.',

				'PluginUserSettingsEditAction' => 'Called as action before editing a user\'s settings.',
			);

There's doc pages for them all, but buggered if I can find them, I find it easier to read code ;)

The hooks you'll ( probably ) require are ;
'DisplayRegisterFormFieldset' => 'Called when displaying the "Register" form.',

'RegisterFormSent' => 'Called when the "Register" form has been submitted.',

¥

You have to develop it or [url=http://forums.b2evolution.net/viewforum.php?f=34]post a request[/url] and wait.