You need to use the scheduled task. There are two good libraries you can use:
Quartz.Net and
Bookmark Scheduler .
With the help of the scheduler, such a task becomes easy. You just create a task that runs after processing time. There you can check any unprocessed requests and, if there are any, you will notify about the next set of people and set another scheduled start after the end of the processing time.
Let me know if you need more information.
I used Quartz.net in azure webrole successfully in a production application.
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