Head count formula not working

HI, SofG 

Please try this formula:

measure = 
VAR tmpCalendar = ADDCOLUMNS('Dynamic Calendar',"MonthYear",VALUE(YEAR([Date]) & FORMAT(MONTH([Date]),"0#")))
VAR tmpBilling = ADDCOLUMNS('People Data',"MonthYearBegin",VALUE(YEAR([Employment start date]) & FORMAT(MONTH([Employment start date]),"0#")),
                                      "MonthYearEnd",IF( [Employment end date]=BLANK(),VALUE(YEAR(MAX('Dynamic Calendar'[Date])) & FORMAT(MONTH(MAX('Dynamic Calendar'[Date])),"0#")) , VALUE(YEAR([Employment end date]) & FORMAT(MONTH([Employment end date]),"0#"))))
VAR tmpTable = 
SELECTCOLUMNS(
    FILTER(
        GENERATE(
            tmpBilling,
            SUMMARIZE(tmpCalendar,[Year],[Month],[MonthYear])
        ),
        [MonthYear] >= [MonthYearBegin] &&
        [MonthYear] <= [MonthYearEnd]
    ),
    "Persion ID",[Person ID],
    "Year",[Year],
    "Month",[Month]
    
)
RETURN  COUNTAX(tmpTable,[Persion ID])

If you still have problem, please share your expected output with little sample data.

Best Regards,

Lin