How to identify a bunch of identical ARs?

[N7XW]

Use the Army method and make them all memorize their serial # 

[Yondering]

You could get a different Magpul (or whatever brand) grip for each person, they are cheap enough to make good stocking stuffers and are one of the first things I prefer to change anyway. Lots of different colors and shapes available; I prefer the K2 grip but the MOE is fine too, and the original K is good for people with small hands. All of them come in a number of different colors, even pink.