Talkaudio has a section like this. It called 'Regular members only'.
To get in you have to be on the forum for over 12 months and have over 500 posts (or something like that).
Back in the day it was full of pics of users girlfriends and general smut, but now is more for legal advise and discussions that users need help with that they don't want out in the public arena.
There is also a MO code that is basically what happens in MO stays in MO, any leaks and the leaker is out.
Traders will also post special deals in the section before putting them up publicly.
its really cool to have a inner sanctum like this IMO.