A party of 5 will get 1.5x Exp from any monsters. So let's say one monster gives 50xp, in a party a 5 you than get 1.5x which is 75xp.

Now divide it by 5 and you get 15xp each. Doesn't seem like much, but then you need to realize that all depending of your classes, you are killing alot faster than normal 1 player alone. If I take example on my 5 shamans. 1 Lightning bolt each would kill pretty much anything one shot, so faster kills, less resting time, if any. But I have to admit, instances are the way to go in that set up. Great loot, lots of green to DE and cloths.