You won't necessarily need after effects, but some kind of compositing tool, and most of the available info online seems to center around AE...
Bunch of time, handful of people, and a chunk of greenscreen:
https://www.youtube.com/watch?v=6rJ8rJREAIA
Or if you've got AE & Trapcode particular (or form):
http://www.redgiant.com/videos/redgianttv/item/26/
This Lynda course:
http://www.lynda.com/After-Effects-...Crowd-Replication-After-Effects/114012-2.html
And of course, if you have access to massive, there's a stadium agent available...
https://www.youtube.com/watch?v=2JPFFwFMoSU