It would probably work within the PM system that will come around (at some point)
Having a private discussion available to certain users would (I think) do what OP is asking. I don't think groups were discussed, but would be a useful feature of it.
I'll see if I can find that thread..
Stealing from jordanjay29
FYI, there are a couple threads on this discussion if you want to get ideas from the community. Here and here.
This would allow you to, say, create a thread so that only you and users in group Moderator could view / reply. Or you and user X Y and Z