Once you have created your league you can now add your teams.
Within your league select 'Teams'.
Select '+ Add Team'.
The following screen, you will need to give your team a name, select a team color and then you have the option to keep the registration open or closed.
If you select open, then participants will be able to register and select the team they wish to play in. If you keep the team closed, then once the participants have registered you can then assign them to a team.