It's a community site but I'm not sure how to promote from the beginning. If a site has very few registered users it's very difficult to get new users sign up.
Yes, it's the BIG problem when you start an online community. I really think the more important is to work on SEO to have a good ranking. It's an hard work but necessary if you want traffic from search engine: traffic = visitors = registered members.
First it's necessary to patch the joomla core. Joomla Patch extended do this work very well. Then, you can apply <h1> tag with all content titles. There are many other important things to do to improve SEO in Joomla! When you think about visitors, don't forget the important work you must do for SE ranking.
You can also use a "recommend" or "tell a friend" module (search for ajaxrecommend at sakic.net) and/or the "plaxo invite" commercial plugin for CB. These tools let your visitors and users invite other peoples to join the community.