Q u o t e: http://us.battle.net/d3/en/forum/topic/4856946388?page=2#21
One of the interesting things that happens, especially when your group gets up to four chars, is that it becomes more and more likely that you'll be dealing with extra monsters because it's so difficult to keep everyone tightly grouped together.
It's not usually that big a deal, but when your group aggro radius gets big *and* you wander into a situation where you manage to get two (or three -- I've actually pulled three) packs of champions on you (also because the group doesn't manage to kite perfectly back into completely cleared areas), well... you can imagine how it all ends up.
One of my minor predictions is that when you see a video of a Hardcore team clearing through the later difficulties, if they're any good, you'll see them sticking very close together.
Nice. Apparently it's possible to run in to 3 packs of champions in later difficulties.