BuddyPress.org

in the function groups_post_update a check is performed if someone is a member prior to posting an activity_update (there is no action hook prior to this conditional) – so you’ll need to find out how to join all your members to groups first. (another plugin or custom hook when someone registers, creates a new group, views a group, etc)

the theme for /groups/single/activity.php contains a check for bp_group_is_member() around the post-forum.php include