[Xion]

On Inferno the monsters scale quite massively with each and every player that joins the game. I believe it's 110% of hit points and about 15% of damage per player.

So your party have to synergize alot in order to overcome this scaling. This is further exacerbated by the Nephalem Valor buff's mechanics - a bonus to magic find you accrue after killing hard mob packs - which prohibits changing your skills and runes mid-game. Hence you cannot really have your party members leave and be replaced by someone of different class without losing the synergy (or the buff).

In solo play, of course, you don't have these problems.