How do I get the count for time periods?

I have a table with the following columns.

[ComputerGroupid], [Computerid], [Userid], [loginTime], [logoffTime]

And my requirement is to get the table with the below information

[ComputerGroupid], [StartTime], [endTime], [numberOfcomputerBeingUsed]

But I am not sure how to merge the date ranges when they overlap and also when the number of computers being used are equal?

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edit: Some more details. What I am looking for is to get a table of computer usage for a group of computers. so I am expecting to create a table where when ordered by time the [numberOfcomputerBeingUsed] column goes up as people log into computers and the [StartTime] and [endTime] columns will match up.

eg: if 4 people come into a room and login at about the same time, for an hour, and one person comes in and logs on for 5 mins and then leaves. I would expect:

 [ComputerGroupid] | [Computerid] | [Userid] | [loginTime] | [logoffTime]
 A                 | 1            | 1        | 09:00       | 10:00
 A                 | 2            | 2        | 09:01       | 10:01
 A                 | 3            | 3        | 09:02       | 10:04
 A                 | 4            | 4        | 09:03       | 10:03
 A                 | 5            | 5        | 09:10       | 09:15 

  [StartTime]  |  [endTime]  |  [numberOfcomputerBeingUsed]
  09:00        |  09:01      |  1
  09:01        |  09:02      |  2
  09:02        |  09:03      |  3
  09:03        |  09:04      |  4
  09:10        |  09:15      |  5
  09:15        |  10:00      |  4
  10:00        |  10:01      |  3
  10:01        |  10:02      |  2
  10:02        |  10:03      |  1
  10:03        |  10:04      |  0