You can join any group, however only we can start groups. This is simply to avoid random groups starting even if there are already similar groups. If you have an idea for a group that is not being done awe are happy to add it!
Just go to the group page (the groups home page is linked under the "business" tab in the navigation bar. Go to the homepage of the group you wish to join and click the "join group" link (second column, right part of the page).