How do you keep members from choosing more than one group?

[tgreer]

I see no setting for this. You'll have to code a plugin. Looking at "profile.php", the "joingroup" code starts at line 1999. There is a hook at line 2052, the hook is called "profile_joingroup_unmoderated". That comes right before the $userdata->save datamanager call, so presumably you could code a plugin that does a query, checks to see if the user is a member of any other usergroup, and if so, do a redirect to an error page so that the codepath never reaches the "save".