BuddyPress.org

You should probably look at the “Auto Join Groups” plugin:

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

It does what you want it to do, but automatically from the admin area.

Haven’t tested this in BP 1.2 though.

Might want to contact the dev of that plugin if it doesn’t work.

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Also it is quite easy to modify DJPaul’s Welcome Pack plugin to auto-join groups as well.

See my post here:

https://buddypress.org/forums/topic/bbpress-and-buddypress-1#post-43925

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Update:

Just came across a plugin called “BP Registration Groups”:

http://hardlyneutral.com/wordpress-plugins/

Purports to do exactly what you want, Bowe.