BuddyPress.org

You’ll have to know some coding to do this. This will get you started if you know some code.

You can add the hook to the following:
add_action( ‘bp_core_signup_user’, ‘join_all_public_groups’, 10, 3 ); //For WP
add_action( ‘bp_core_activated_user’, ‘join_all_public_groups’, 10, 3 );//For WPMU

Then, in the “join_all_public_groups” function, you can use this call to have them join groups: groups_join_group( $group_id, $user_id ); You’ll just need to loop through the list of groups.