Carlos, so you're looking for how we usually approach creating many to many tables?
The relationship is (role)1..N to 0..1(groups)
This would usually imply that you have a 1 to many relationship, not many to many. I assume this is just part of the language barrier.
In this case, because your role_group_xtbl will not be containing any additional information, I would simply PK across the two ints. Depend on which direction you'll usually go (I assume your typical parameter will be role, which will then transit to find out what group permissions it has) I'd lead the table with Role_ID so you can seek directly to your target rows when you build the clustered index.
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