Forum Moderators: open
People hiring SEO's, web designers, purchasing dedicated servers or hosting, etc... etc...
If only WBW got 1% of all "deals" that happen backstage, that would be a nice little bonus I think and keep the site well running.
I don't know how you would enforce it, maybe just a kind reminder or suggestion somewhere in the stickymail center, out of the thanks for bringing internet professionals together in an accessible manner.
Why not make a "teaser" page when people that are unregistered for the private forum try to access it?
Like list all the topics but none of the content... I know that would drive me crazy enough into getting a membership :)