In order to shoot that amount of people, its a good idea to get high and far away. On top of a roof, up in a manlift, something like that. 50mm is about what the eye sees on a full frame camera so 100mm is 2x "closer" than what the eye sees. The longer the lens, (more than 50mm) the further back you'll have to be. The shorter the lens, (less than 50mm) the more distortions (stretched faces) you'll have showing especially at the edges. You can do this a number of different ways, each is going to force you to choose the lesser of the evils. I would personally prefer to shoot at 85-100 to minimize distortion, but only if you can get far enough away. Also, unless you want to do a bunch of math to calculate hyperfocal distance, (or simply download the app) shoot with a closed down aperture, like f8-f11 or more to get the depth of field you'll need so the far faces aren't blurry, but that is also going to force you to shoot in bright conditions so your shutter speed can stay high enough to account for people moving. Try focusing on a person a third of the way into the group. That will maximize your depth of field as much as possible if you need to open up the aperture for poor lighting conditions.