Well, maybe "private" is not the best term. I mean that a special page similar to "Discussion boards" page will be created and any member of Brain Rook or higher level can create new discussion board(s) and set some restrictions for it [for example, to make it readable for everyone but writable for selected users only].
Of course, the board creator will be responsible for the board moderation. Boards with an unacceptable content will be deleted etc.
How would you define a "group" to dominate the fellowships?