To join a team, first open the Team menu.
You can find it here:
The following menu shows the existing teams. You can use the search menu to find a particular team, or use the arrows at the top right of your screen to browse through existing teams.
Teams are either "open" or "restricted"
If a team is open, you can join it freely. Simply click the team in question, then click "Join".
If a team is restricted, you need to know the required password to join it.
The numbers next to each team indicate their current amd maximum number of members respectively. For example, 25/50 means that the team currently has 25 members, and can have up to 50.