Within Teamwork, you can break your teams down further by creating subteams.
Note: Subteams are available on the per-user Premium plan and above.
When you are creating a team, you can make it a subteam by assigning a parent team to it in the Advanced tab.
Alternatively, click the three dots to the right of a team in your teams list and select Add subteam from the dropdown menu.
When a user is added as a member of a subteam, they will automatically be added as a member of the parent team and any other subteams above it in the overall team hierarchy.