For anyone else, found the solution to my issue.

member [Measures].[NumCustomers] as
        count    (
                    filter    (
                                 exists([Customer].[Customer Key].[Customer Key].members , {[Customer_Purchase_List]})
                                , (
                                     [Date].[Date].[All].[AggRangeOther]
                                    , [Measures].[Sales Amount]
                                 ) > 0
                            )