I would also like to see Andrew’s take on creating armies or large amount of people.
If you’re planning on battle scenes that go on the background, you’ll need to have 2 people fighting in many different styles. (Like 4 or 5 different combinations) and then put them on the background either by hand or by using the 3d script distributor found at aescripts.com. You also must randomize their start and end times so that they don’t look like robots. Now these are already in the article you linked to. But a few tips may help.
This method works well if you plan the shot. If you use high angle, it will become much harder to use this method because the greenscreen footage would also need precise angle calculation to go with the high angle shot. So you should shoot the video from eye level or a bit above eye level.
If you only plan on building an army that just stands there or marches somewhere, the difficulty depends on the way you shoot the scene. For example, if you plan on moving the camera sideways facing the army, you can’t just have the person shot directly. Because that way you wouldn’t be able to see the edges of the actors that pass by. You would need greenscreen footage that is well-matched with your shot. These are the most amateurish ways I suppose.
If you want armies marching and camera moving freely, than that would require 3d programs.