There aren't any settings within the tools right now that would accomplish what you are trying to do. You would likely need to do a bit of custom work to achieve an effect that is close. Spruz does some custom development work, and may be able to do things exactly to your preference regarding this. You can ask them in a support ticket how much it would cost to implement the system for you.
As a partial fix, you could just make the groups page be the page that everyone is redirected to after they have registered, and then maybe put a rich text editor element at the top of the groups page that says "Please join at least 1 group". Maybe that would work just as well for you.
