Forum Need to set special forum permissions based on user level...
 

My head hurts trying to figure this out. Here's my dilemma.

I'm offering a member of my site a private forum for a special event he's having. Most of his attendees are already members of my site so they're not a problem to add to the private forum. However there may be some people who are not members and do not want to join my site and I'd still like them to be able to use his private forum. My site is a pay site so some people may not care to join.

I have the private forum set up.... changed current members forum permission (access mask) to allow them to see and use the private forum.
I have a free guest membership level set up as a default level in which paid members will automatically get downgraded to if they forget to renew. This free level has no viewing/usage ability across the site.
I thought I could use the guest level and change a few permissions and allow individuals access and use of ONLY his private forum and NOT the rest of the regular forums. This is not working as I hoped.
Once I make changes to the forum permissions, they are site wide.
I hope I was clear enough.

Any thoughts or ideas would be greatly appreciated.
Thank you