Is there any way to make a resource mailbox visible to a specific user group only?
We've got a meeting room which should be only bookable and visible by a small group of persons. Unfortunately it's not an option to just hide it in the adressbook because this way there is no possibility to check the rooms calendar.
Not easily.
You would have to create a new Address Book Policy for everyone - one policy that allows that account to be seen, one that doesn't. Then apply the appropriate policy to each mailbox.
If you lose the requirement to be visible, then restricting the use is easy enough - you can just permissions on who can view and book it. It will appear in the list, but correctly setup it will not allow anyone to book.