The Poptropolis Games may be gone, but you can still join and leave Tribes to your heart's content. Just go to your Friends profile page and click on your tribal icon. If you're already in a tribe, press the "change tribe" button and you can choose a new one.
No matter what Tribe you're a member of, you can enter that Tribe's exclusive common room. Each one is different, so check 'em all out to figure out which Tribe is the best fit for you!