Why not create a special forum for those interested to join, and it be a special group set up? I belong to another board (it's pagan/wiccan related) and we have a special forum set up for a special group called 'Elders'.

It's a forum created for 18 and over members who have had x-number of years following their beliefs. Basically, members applied to join and had to be approved before joining the group. Or were invited to join by site administration. The forum itself isn't viewable to other members, just those with the 'Elder' attachment to thier membership.

What could be done is set up a new forum, have those interested in participate 'join' the special forum, yet keep the forum itself viewable to all members who then can join if they want to.

As to the moderation of it? Normal moderation would be the best. Assign one or perhaps a moderator could volunteer to watch over that particular section.