Not sure if there is such a function on this forum but on a previous forum I used for a online RPG game we had different section within the forum where only certain people would have access to, i.e a basic public section, a guild member only section, an officer section etc etc, Someone with administrator access to the forum would be able to grant members access to relevant section. Doesn't stop people posting in public section of course but you can discuss matters in private that may concern.
