You can leave any team you are a member of and you can join any team that sends you an invitation or accepts your join request.
You can control what teams you are a member of by joining or leaving those teams as is appropriate. Currently this must be done from a browser on a Laptop/Desktop computer.
There are two main ways to join a team. One is to have someone with an Admin or Coach role within that team invite you to join. That will send you an email notifying you of the invitation and you can accep that invitation in your main Team Settings page as seen below.
You can also request to Join a Team in the top right of that same page. This will allow you to search teams by name and request to join. Once you request to join, someone with an Admin or Coach Role on that team must accept your request to join in the Manage Members Section of the team's Team Hub.
To leave a team you must go to your main drop down menu and into your Settings. Inside your Settings, select User Permissions from the left side menu to open the page below.
Here you will see all your current team memberships and some Privacy options. Below each team you will see an option for Leave Team. Simply click on that and then confirm you want to leave team.