If you're looking for a busy, well trusted forum to join to share porn and interact with likeminded people, may I suggest Forumophilia. This site is based on generic forum software which makes it extremely easy to use. The homepage shows a number of groups and boards which you can actually view for free!
If you want to comment on or start your own topics, you'll need to register for a free account but the amount of options here are pretty impressive. There is the teen babes fanclub for all women 18+, popular models, pornstars and celebrities and even full movies too.
This site has over 290,000 registered users and there is always a lot of people online at one time. New forum posts are constant and there's a VIP section if you have posted more than 300 posts... I wonder what's in there!?