I’m not an expert, but I think I know how you could do this. First you need to key out the green screen so you have your alpha channel and that one person. After you have your one person, create a comp that is bigger than 720×480, (probably a lot bigger) and copy and past that person across the screen in your comp. That should give you a row of people doing the exact same thing, so you could call that comp “row of people”.
Then create a new comp that is 720×480 (we’ll call this the new comp). Put in the “row of people comp” inside this new comp. You then can position the row of people so that this would be your first row of people at the front. Then duplicate “the row of people” comp again inside your new comp so that there are two, turn down the scale on the second one, and put it higher up so that it creates another row of people that looks like it is behind the first row. If you want even more rows, you keep duplicating the “row of people” comp and turning down the scale and putting them higher on the y-axis to make it look like they’re farther away.
I hope that this helps. Just let me know if you have more questions. And anyone, please feel free to correct me if there is a better way to do it.