Answer

Step 1: Create a WINDOW_SUM Calculation

1. In Tableau Desktop, connect to Superstore sample data.
2. Select Analysis > Create Calculated Field.
3. In the Calculated Field dialog box that opens, enter a name for the field. The COUNTD Alternative example workbook uses Distinct Customers.
4. In the formula field, type the following, and then click OK:
   WINDOW_SUM(MIN(1))
5. Drag Distinct Customers from Measures to Columns.
6. Drag the dimension for which you want the distinct count from Dimensions to Detail. The example workbook uses Customer Name. 
7. Right-click Distinct Customers on Columns, and then select Compute Using > Customer Name.

Step 2: Filter on Index

1. Select Analysis > Create Calculated Field to create another calculated field.
2. In the Calculated Field dialog box, type a name for the field. The example workbook uses Table Calc Filter.
3. In the formula field, type the following, and then click OK:
   INDEX()=1
4. Drag Table Calc Filter from Measures to Filters, and then click OK (do not tick any available option).
5. Drag Product Category from Dimensions to Rows.
6. Right-click Table Calc Filter on Filters, select Compute Using > Customer Name, select True, and then click OK.

Additional Information

• If Tableau Prep cannot be used, see: Getting around Count Distinct in a secondary data source (third party website)
• Creating a primary group: Bring a Field into the Primary Data Source in Tableau Help