SQL Server Central continues a look at PowerPivot’s DAX language with Stairway to PowerPivot and DAX – Level 3: The DAX DISTINCT() Function and Basic Distinct Counts
Stairway to PowerPivot and DAX – Level 3: The DAX DISTINCT() Function and Basic Distinct Counts – SQLServerCentral
Archived post from 2013. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.