Before you can join a team, you must first have your password (see How come I haven't received my password?). Once you've gotten your password, and you've successfully edited your participant information, then you'll be in a position to join a team.

The simplest way is to simply go to your team's stats summary page and click the "I want to join this team" link. You'll be presented with a login dialog box where you should enter your email address and your password.

That's all there is to it!