Groups on the PRO subscription can create internal Teams. To see all the Teams in your group, and join one, just go to your Settings>Teams page. If your group has Teams, they will be listed, and you can join or leave simply by clicking the button to the right of the team name. If your group allows "user generated" teams, you can create a new team by clicking the "+ Add Team" link at the top right of the teams list.
If you do not see this option, but know Teams exist in your group, it simply means your Admin has restricted those privileges to just themselves & other Admins and will be assigning you a Team once you are actively registered to the group.