Can anyone recommend a simple message room (not chat room) which I can use for a private communication between my users.

My idea: to grant some users (2-3 at the most) a specific privilege to talk privately in a message room. none of them would be administrator there.

I need such features:
- admin panel for adding users allowed to post messages in room
- room invisible to anyone except users
- filtering not-allowed words
- Ajax-enabled so that replies appear immediately
- other message room features

I have a linux hosting so PHP message room would be the best option.

I thought of using a forum for this, but it's too much work as a forum is public by nature and I need something private by nature.

Any ideas?

Thanks in advance.