From your homepage, point your cursor to the top where it says Groups.
A drop down menu appears. If you click on All Groups & Teams you can select which one you want to join.
Once you've decided, click on the name of the group, this takes you to that groups homepage where on the right side of the page it shows 'Join Group'.
You can join as many as you want. Some groups you can join immediately, while others ask you to send a message to the group creator saying why you want to join their group.
Once you've joined, you can then play in team matches from the group.
how does one join a team here