There was some code posted for this a few weeks ago, a quick search should unearth it. What you'll need to do is edit post_info.asp to check if the member's post count is 10 after they make a post and add them to the allowed members list for the required forum if it is.
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Search is your friend “I was having a mildly paranoid day, mostly due to the fact that the mad priest lady from over the river had taken to nailing weasels to my front door again.”