A couple of items needed to grow a community website/forum is to offer needed content and content should be the primary focus. What are you going to give members to read that they can't get elsewhere. What is making your site "special"
After you get your content up...spread the word about your site (no spam) Make sure you put a link in your sig. on all the forums you visit. Buy some banner space on related sites...if your site is dealing with a local issue/theme...contact the newspapers and local websites (they are starving for content for their own sites)
Then the wait game begins. Be polite to the people that start to join. Give them a reason to stay and to pass your info along to others.
If all else fails...try the sex favors