BuddyPress.org

One other idea. In the link to sign up, why not include the group number (or list of numbers) so that the registration theme page could be customized with a special welcome based on the group that invited them without having to do an extra query to the database to see which group they’ve been invited to join.

Something like: http://domain.com/register/accept-invitation/test2%40test.com/groupid=12