I say let 'em lurk. I always hate when forums won't let you see any content without being registered. I don't think I've ever registered for any of those. What's the point if you don't know what the forum is like that you're even registering for?
I probably wouldn't have blindly registered here - it was the still relatively small group full of enthusiasm and excitement for bikes and just enjoying riding, and that it was a large group of local people, that made me register here.