If you're interested in joining a community, simply browse to that community and click the join button below the community photo. If the community is public, you'll automatically be added to it. If the community is moderated, the community moderator will need to approve your request before you are actually added to the community.

If you'd like to search for communities on orkut.com, click Communities at the top of any page or select manage in the my communities box. Use the box that appears on the right to search by name or browse by category.