BuddyPress.org

Try one of these:

https://wordpress.org/extend/plugins/auto-join-groups/

http://brentandmary.net/2009/10/24/bn-auto-join-group-plugin/

http://buddypress.webdevstudios.com/blog/2009/11/13/buddypress-registration-options/

https://wordpress.org/extend/plugins/welcome-pack

I think that one of them does what you’re describing or could be modified to do what you want.

As an added bonus, here’s one that should subscribe people to a blog at sign up:

https://mu.wordpress.org/forums/topic/13675