One use case relates to sending people a link to migrate to your forum–Facebook, Meetup, etc. You don’t have their email, but you need to assign them to a group or at least let them pick a group.
Options for new feature(s):
1/ let the user pick group(s) on registration
2/ change invite feature to not require token per user so you can generate general link to published to users with groups associated with invite
3/ allow users to be added immediately after they click the “join group” button on a group page or on /groups and then register/activate or login
1 would be trickier from a UX, custom field, standpoint. It covers the, “I just showed up at this site” scenario and i need to pick a group.
2 would allow assigning users to public and non-public groups via email invitation.
3 seems to cover various scenarios; discussed here and possibly already on @tgxworld list?
Personally for me, order of priority would be 3,2,1.
If #3 gets done you could direct users to register via the /groups page. Could also have a setting so the signup link is changed to /groups. This would only support one group without more changes, but if the user is returned to the /groups page they could then choose group 2, 3, etc.