This feature is available to all customers on the "Platform" plan.
To create teams in your account, click on Admin → Teams, and then click the [+ Add Team] button.
Enter a team name and select which users should be part of the new team, then click [Save].
The main purpose of using teams is so that you can designate one or more members of the team as a team leader. Team leaders have access to all leads assigned to any member of the team, and any leads a team member is a collaborator on.
When on the team screen, you can select which members of the team should be a team leader by using the checkbox under the "Team Leader" column: