Effective Dated User Table with manager Information

Anonymous,

This solution requires a slight change to the User table. If you restructure it to use Start Date and End Date, it simplifies the DAX:

Create a bridge table using DAX (calculated table):

DistinctUser = DISTINCT ( User[User] )

Create relationships as shown below. If you prefer not to use a bidirectional relationship between User and DistinctUser, you can use USERELATIONSHIP in the measure (and specify bidirectional).

Use either of the measures below (you can try both to see which is more performant):

Count CALCULATE = 
VAR vStartDate =
    MAX ( User[Start Date] )
VAR vEndDateCol =
    MAX ( User[End Date] )
VAR vEndDate =
    IF ( ISBLANK ( vEndDateCol ), DATE ( 9999, 12, 31 ), vEndDateCol )
VAR vAmount =
    CALCULATE ( SUM ( FactTable[Count] ),
        FactTable[Date] >= vStartDate,
        FactTable[Date] <= vEndDate
        )
// calculate total
VAR vResult =
    IF ( HASONEVALUE ( User[User] ), vAmount, SUM ( FactTable[Count] ) )
RETURN
    vResult

Count SUMX = 
VAR vStartDate =
    MAX ( User[Start Date] )
VAR vEndDateCol =
    MAX ( User[End Date] )
VAR vEndDate =
    IF ( ISBLANK ( vEndDateCol ), DATE ( 9999, 12, 31 ), vEndDateCol )
VAR vAmount =
    SUMX (
        FactTable,
        IF (
            FactTable[Date] >= vStartDate
                && FactTable[Date] <= vEndDate,
            FactTable[Count]
        )
    )
// calculate total
VAR vResult =
    IF ( HASONEVALUE ( User[User] ), vAmount, SUM ( FactTable[Count] ) )
RETURN
    vResult