I was originally going to post a new topic “how to filter user signup before : after” so all new users from X day go into X group
However, I found this topic and decided it was what I needed.
The difference in my use case is to your example (thank you again) which is possibly even simpler, there is no need for multi chocie so the user only have to tick a “confirmation” 
and that confirmation field fix the users to X group