HELP!! How can a forumla work out the amount of people at any time?

Hi,

Im creating an Excel sheet and struggling on a few issues.

Firstly, i'm trying to create a forumla to show how many people are working between certain time ranges. I have the start time in one column, and the end in another. Is there a way - for example input a time, and see how many people are working at that time?

Also I have about 6 sheets/tabs in one excel document.

So is there a way to show....
How many people are in total working at 10am,
How many on sheet 2 (for example) are working at the same time? etc

Thanks for any help!