You will want to have a Legal Notice and Acceptable Use Policy for your site. If you have one of these then I believe you are pretty protected from libel. To be extra safe, you may want to have your site set up so that everyone who joins has to acknowledge they have read that page.
Here's an example AUP This is a good start, but this AUP is for a site that is completely unmoderated so you should check out the AUP pages for some moderated sites as well, unless you are planning on having a unmoderated site.
Heidi