Joining a tribe will add it to your "Tribes Feed" (one of your 3 main private feeds), where you can stay updated on the content that is posted in it. It also allows you to show up in a tribe's leader board, and earns you "Tribe Mastery". With the Snapzu Platinum Member status, you will also have post counters on your 24 most active tribes where new activity will be shown to you in you Popular Tribes overview area.