Hi Ice,
I never thought about this kind of feature, beacuse usually it is a good practice to leave "Registered" group assigned to registered users, and add or remove additional groups as needed. This is useful to avoid potential issues related to a missing "Registered" group.
Anyway, if you need to do this I just need some additional information to help you and find a fast solution. So, AG works assigning (and unassigning) groups based on the value of a field. You could get the result for "Registered" group only if you have a field value dedicated to this group. A line like this could solve:
fieldname;Registered only;Registered;true
If you need (as it seems) to just unassign the registered when another group is set, we need to customize the AG behaviour, but I need more details (e.g Is the user that can change his group? What happens when the user leave the group?).
For these details, if you prefer, you can send me an email at info(at)selfget.com.
Kind regards,
Luigi