This is a quick hack rather than a plugin. It allows users to participate in the forum discussions of public groups of a BuddyPress based network without joining the group. If you are on BuddyPress 1.2.9, you even don't need this plugin. You can simply put
define( 'BP_DISABLE_AUTO_GROUP_JOIN', true );
in the bp-custom.php and it will disable auto group joining for public groups.