While enemies grow in power with each additional party member that battles nearby, the rewards heroes receive from working together to defeat these foes also increases. In addition to bonus chance to find magical items and bonus gold received, you also receive bonus experience. This experience bonus is multiplicative with difficulty bonuses, so playing multiplayer at higher difficulty levels yields even greater rewards.
2-player game: 10% Bonus to Gold Find, Magic Find, and Experience
3-player game: 20% Bonus to Gold Find, Magic Find, and Experience
4-player game: 30% Bonus to Gold Find, Magic Find, and Experience