The Tableau UserSync Tool is an add-in managing your Tableau Server’s or Tableau Online User Accounts, Groups and Group-Memberships.
Tableau supports several types of Authentication. Most known are ‘Local Users’ and ‘Active Directory’. When Active-Directory (AD) is used, most of the time Tableau is able to import users from your domain-controller. Also AD-groups can be imported, including its members, and Tableau can schedule a regular synchronization with your Active-Directory. That’s a good feature of Tableau Server / Tableau Online.
Almost implicitly, it is assumed that your domain-controller, with Active-Directory, resides closely to Tableau machine, in the same network segment. But the above prerequisites are not always fulfilled.