Your Teams subscription automatically includes seats for 3 users, but you can purchase additional seats if need be! You will have the option to purchase additional seats directly from your Projects page within your Team Workspace, or on the Account page.
You can find the option to add seats next to your Team Member avatars on the upper right side of the Projects page. Click on the + icon to add a new Team Member.
You can also head over to the Plan tab on the Account page. Click on Add Seats and from there you can choose how many additional seats you want to add.