Engrams drop for random players, just like ammo. It has NOTHING TO DO with who killed the enemy, or who did more damage, etc. Its random once the "engram roll" has been made as to who gets it. I have seen have my teammates kill enemies and watch the green/blue ball drop . Last week I got a purple and no one else saw it. Don't know who got the kill, it was hectic, and maybe at a glance saw it and thought it was purple heavy, so I forgot about it till after the fray. They were happy for me when I told them what I got (it turned into ascendant mats, btw)