Whoever gets the killing blow gets full EXP but everyone else on the floor, within a very very VERY sizable range, gets 75% of the kill's EXP. Its more or less about people getting EXP even if they dont get the kill or damage enemies. The range on this is very big, it can sometimes even cover the entire floor that you're on depending on the area (Forest 2 for example). It's very useful for quests with split paths on the same floor as well, since none of you will be missing EXP drops from enemies your buddies kill 7 rooms away from you.