There's a plugin floating around for BuddyPress that links profile fields to groups, so as a user signs up, it will auto-join groups for them based on conditionals.
IE: Profile field "Male" or "Female"
if male => auto joins group "male"
if female => auto joins group "female"
It worked for previous BuddyPress versions, but no longer works, and it appears support for it has died.
We found it pretty useful, and will dig into it and fix it if need be, but thought i'd see if there's interest here first.
Thanks as always.