1. Set Require Registration to On in Admin Options->Main Forum Config 2. Enable Private forums in Admin Options->Feature Config. Then edit the desired forum properties and set it's Auth Type to Allowed Member List (hidden) and add all the members you want.
P.S.: You should have posted this in the General Help forum.
If you set all the forums in a category to Allowed Member List (hidden) the category itself will not be shown to those members who do not have access to any of the hidden forums inside. So I would say that, indirectly, something can be done.