Happened to me atleast twice. Not sure if it can drop from a group of mobs that you didn't damage at all (or were close to them while being damaged) because usually when it happens it's like when you hit a few mobs then you continue and your friend stays behind and kills a few trash mobs and then you see a star quite far away from you.
I mainly ask this to see if it would be efficient for me and another player to say, split up at the top of a map and meet somewhere in the middle for XP and legendary farming for the boosted group xp and mf. we have tried to play together but my crit rend barb steals all the kills and makes it almost impossible for my friends DH build to work because he needs large groups.
You only count as "together" and therefore share loot and XP if you are close together. I dont remember exactly, but i think the area is approximately a circle that touches the minimap square from the inside. Try it, it's really small, maybe 1-2 screens away.