If the purpose is somewhere to work out problems without outside interferance, this can be done off the boards entirely.
If the purpose is to hash out differences providing a record for everyone to see, other posts should not be allowed at all, and I woudl go so far as to say it should be done in a private forum, and then posted later once things are resolved.
If the purpose is something else, that is an area to work out problems that don't seem to be workable where they originate or off the boards, then not only does it not make sense to limit the participants, but outside participation should be both desired and encouraged. Most unresolved conflicts only exist because both sides refuse to see the others point of view, and to resolve that conflict, other input is necessary.
It makes no sense as is, it's just like a public email area where everyone can see your fight.