Graph with sum line

This is a classic problem that I have seen a few times, the solution is to create an "active employees" measure with a disconnected Calendar.

To do this you will need a Calendar table with contigeous days in it.

The fastest way to do this is create a new table 

Calendar = CALENDARAUTO()... this will scan your data and build table with a list of dates. I recommend you build a proper date dimension but this does the trick.

This Calendar[Date] is used to slice the data, no relationships are needed as we build a measure to lookup the value.

Then your employee count measure:

Employees = DISTINCTCOUNT(Table1[EmployeeID])

Then you need a measure that counts the employees whose "admission date" is before (<=) the last date in the selected period and "termination date" is after the first date in the selected date period

CALCULATE([Employees], FILTER(Table1, (Table1[Admission Date] <= LASTDATE (Calendar[Date])) && (Table1[DismissalDate] >= FIRSTDATE(Calendar[Date]))))