compositing and careful blocking is the answer. In the old days we might have called it a "split screen", today it's just a "comp". basically what you need to do is imagine your frame divided into 10 sections. In fact, if you have a video tap you can mark right on the screen with a dry erase marker (only on a glass screen!) to make each setup a little easier. Lock off your camera and place your 10 subjects in the first section and roll for the duration of your intended shot. Then move all the subjects to the next section and roll again. Do this ten times and you have 10 shots of 10 people. Now the tricky part. After you capture your video, you will need to bring it into a compositing solution. Most editing packages have rudimentary masking ablilities but to really make it work use a package like Shake, After Effects or Fusion. Start with the first take as your background and layer the second take on top of that using a mask to only show the section of the frame with the people in it. Then layer the third take and mask it as well. Do this for all the takes and the result will be a very large crowd with accomplished with a small group of people. Things will definately get more complicated if you have the poeple over lap each other. In that case you would need to use either a blue screen or green screen and extract mattes. This is a very tricky endevour and should probably be avoided if you are not familiar with keying. Some tips would be to have the people change costumes or mix things up a bit between takes to make the crowd more random. If the people are moving in the take, place small markers on the ground and use a stop watch and vocal commands for position and pacing. Use a heavy duty tripod and sandbag it to keep it absolutetly still. Have fun and make some movie magic!