I'd use a platform, crane, ladder, anything to get higher in order to shot down. Otherwise the picture might look like a banner. Unless you can set them up in a stairway, but I doubt that you can get 500 people to stay still in a stairway...
Now if you could use a drone, you could set up boundaries on the ground, install them and have the drone go up to take the shot. If I was getting paid to do such a shot, I would go this route even if I'd have to rent one with the pilot.
Hope this helps cause you're going to need all the help you can get.