You can join as many groups as you want. Check out my page; I'm in all of them. I'm not quite sure why you're having a problem hitting the tab to join the group. That's all you have to do. You may want to check your spam folder for a little while because that's where they go when you first join CSRnation.
If you're looking for more work, don't forget to hit on the Cover Depos tab on the top of the main page, hit the red dot on the map where you'll cover work, and then click on the tab to "Join the Group." Once you do that, your email address will be saved with everyone else's email address and you'll receive email blasts of jobs in your area. You just contact the agency directly from the email with their contact info.