If you choose to have a private subforum, you might restrict it to members who have a minimum post count. This can be beneficial as it will inspire current forum members to post more and might increase new sign ups. However, the down side would be how you could also get a lot of people spamming useless (one word) posts just to pad their post count and gain entry to the private subforum. What do you think about post counts for private subforums? Or how do you handle the situation of a private subforum on your own site?