Are you trying to get it that a member would need 15 posts before entering, so that it would be a private forum?
The thing I would do is manually allow them in in the CP, but it is tedious, and if you wanted automation, you'd have to do a lot od editing.. If I were you, I'd stick to manually allowing them in, but I'll see if I can't come up with something for you.
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If I were you, I'd stick to manually allowing them in, but I'll see if I can't come up with something for you. 
