Click the gear icon and select Settings.
Under Admin Settings, select Team Management.
Next to All Teams, click the +.
Enter a team name (and optional description) and click Create.
You can now share templates, campaigns and groups with that team.
Still in Team Management, select the Everyone group.
Locate the users you want added to your sub-team and select their checkbox.
Click Add Selected to Teams.
Click the drop-down and select your desired team(s).
Click Add To Teams when done.