Specifically, the diablo 4 party size is 4 players.
Generally, it is an advantage to group up as party with friends on characters with similar power levels. The life and damage done by monsters in the Open World does not scale with the amount of players. However the life and damage done by Monsters in Dungeons (instances with a maximum of 4 players) scales up with the number of party members.
There can only be up to 4 party members in the same party.
There is one party leader that can add or remove other players.
The State of your Open World State & quest progression depends on the party leader.
Once you join a party your World State is set to the party leaders and you might lose any unfinished quest progression, so make sure you do not accept a party invitation when you are about to finish a quest.
There is a separate in-game party chat to communicate.
Buffs from skills like War Cry benefit party members as well as non allied players nearby in the Open World.
Note that vanquishing demons while near any player will grant you 5% bonus experience and 10% if you are near a member of your party.
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