There isn't always a cloak and dagger reason for people joining and not posting.
Thanks Helen, that's absolutely right. The Forum registration system is also integrated with the site registration system - for instance if someone registers for the online shows they are automatically given a forum membership.
We screen every new member and delete all that are clearly spammers, and put a watch on those that might be.
But many register so they can subscribe to threads and receive email notification, to enter the online shows (but never come near the forum), and many register to post in the Marketplace - all legitimate reasons.
For those who care, we have 6,440 registered members, of which 3,344 have posted at least once. This year we have had an average of just over 1,000 different members posting each month.