That may cause problems though, since I don't really want to create a new usergroup and have to deal with group promotions just to allow users the ability to access a category... not if i can help it....!!! I tried to play with them before and it caused nothing but hassles
If you've implemented permissions in the recommended way, this should be trivial. The new user group would gain no extra permissions, but would gain Node level permissions. Revoke the posting permission on the node for Registered, which should trickle down to all users and be overridden by the Allow for the secondary user group.
Other than a user group promotion, the software has no other way to accomplish this.