When you are logged into your fundraiser page, there is a cogwheel in the left upper corner, next to the button where it says "edit fundraiser". When you click on the symbol, a menu will drop down, where you can select your fundraiser state as well as joining or leaving a team, see below.
Click on "Join a Team", and pick the team you would like to join. You can write the name on the team in the search field, if you know what it is called.
You are now a member of a team.
To see your team, take a look at your fundraiser page under your profile picture where it says "Member of Team".
If you click on your team you get to the teampage, where you can view your team members and how much you have all fundraised, both individually and as a team.