Answer

1. Enable access to the Tableau Server Repository. See Collect Data with the Tableau Server Repository for more information. 
2. ​Open Tableau Desktop and select New Data Source > PostgreSQL.
   • Server: Your Tableau Server URL
   • Port: 8060
   • Database: workgroup
   • Authentication: username and password
   • Username: readonly
   • Password: the password selected in step 1
3. Select the _users table and click Sheet1 to navigate to the worksheet. 
4. Drag the desired fields onto the worksheet. 
5. Select Worksheet > Export > Crosstab to Excel.