I would agree that anonymous posting ability is a great way to kick start a forum. However it shouldn’t be considered a long term option - it will not provide much incentive to join. We actually started small and when it expanded to a point where we could sustain it, we made a few (non-crucial) forums “members only” so it gave them an incentive to not only sign up, but remain logged in. People complained at first, but they get over it quickly. If the site adds value to them, it is worth it to remain logged in. It’s a great thing because newbies see more people online and know the forum is active.