best place for that kind of thing is:
buddypress.trac.wordpress.org
This does it. I have it running on my site, but I modified the code to use a select box instead of check boxes. But this is what you’re looking for, I think.
https://wordpress.org/plugins/buddypress-registration-groups-1/
Or this one if you don’t want them to have a choice.
https://wordpress.org/plugins/buddypress-auto-group-join/
thanks @xkronik13x
it’s the second one that does what I want, I just thought I’d take the opportunity to request features of Buddypress while it’s in a high state of development.
I keep getting told, the less plugins there are, the better the performance.
