The Groups plugin for WordPress is vulnerable to Insecure Direct Object Reference in all versions up to, and including, 3.7.0 via the 'group_id' parameter of the group_join function due to missing validation on a user controlled key. This makes it possible for authenticated attackers, with Subscriber-level access and above, to register for groups other than ones set in the shortcode.
https://plugins.trac.wordpress.org/browser/groups/trunk/lib/views/class-groups-shortcodes.php