Hi anna

If you are using the latest build 2.4.1, find the file [inc/_mian_inc.php]

Find line 360 and look for

 if( !isset($login_required) )
{
	$login_required = false;
}

change the word false to true.

This will ensure everyone has to have an account with password to see any content.

If you want only one blog to be password protected do this to the code: this will protect blog2, which is blog B on the initial setup

if( $blog == "2" )
{
	$login_required = true;
}
else{$login_required = false;}

Just play with permissions;

Create a dummy user and start adjusting their options, you will soon have what you want.

Have fun :)

EDIT

By the way, you could have found this post before you asked :
http://forums.b2evolution.net/viewtopic.php?t=13621 ;)