SQL Server 2012 :: Query To Count How Many Sessions Are Active And Remain Active Per Hour

I have a table with the following columns employeeSessionID, OpDate, OpHour, sessionStartTime, sessionCloseTime. I need to see how many users remain active per hour. I can calculate how many logged in per hour, but I am stumped on how to count how many are active per hour. I have a single table that stores login data. I have created a query that pulls out the only the data needed from the table into a temp table using this query. Also note it is possible that the sessionCloseTime is null if the device has not been logged out this would need to be counted a active.

TABLE NAME #empSessionLog Contains the time stamp data OpDate, sessionStartTime and sessionCloseTime.
OpDatesessionStartTimesessionCloseTime
2015-01-202015-01-20 14:32:59.1302015-01-20 14:33:14.6299166
2015-01-202015-01-20 06:58:33.7302015-01-20 15:27:16.9133442
2015-01-202015-01-20 09:56:22.8402015-01-20 17:56:29.7555853
2015-01-202015-01-20 05:59:18.6132015-01-20 14:05:19.0426707

[code]....

can see how many sessions logged in per hour with the following statement:

SELECT
opDate,
FORMAT(DATEPART(HOUR, sessionStartTime), '00') AS opHour,
Count(*) AS Total
FROM #empSessionLog
Group BY opDate, FORMAT(DATEPART(HOUR, sessionStartTime), '00')
Order BY opDate, FORMAT(DATEPART(HOUR, sessionStartTime), '00') ASCResults:
opDateopHourTotal
2015-01-20041

[code]....

Where I am stuck is how do I count the sessions that remain active per hour until the session is closed with the sessionCloseTime.