Joining a team:

You can join a team at level 50 and participate in the Team Tournament at level 65.

Creating a team:

Once you hit Level 51, you'll have the opportunity to join a team. 
 
Your game will have a button with a heart at the bottom of the screen. Tap it to access the new ‘team’ area, and join, create, or search for a team.

Leaving a Team:

Select the “My Team” tab, then the “Info” button, then tap “Leave” in the window.