Parties allow up to six people to play together. You can create or join public parties by clicking on the Notice Board in town, or by pressing S to open the Social window. You can also invite someone to a party by selecting a player's name in chat, clicking on them in town, or typing /invite playername in the Chat Console.
Effect on Monsters
Monsters gain extra life for each additional party member after the first: 20% in Normal, 40% in Cruel, and 60% in Merciless. The original life amount is used for the purposes of determining the length of Stuns and Status Ailments from Elemental Damage - this means monsters will not be harder to stun/freeze/etc when fighting in a party.
Effect on Loot
Each player after the first adds a bonus +10% Increased item quantity and 40% Increased item rarity to the party. For Gems and Currency items, party members add 50% Increased item quantity and 0% Increased item rarity. Bonuses to Increased item rarity or quantity from modifiers on player gear are only counted from the player who lands the killing blow. This bonus is multiplicative with any player bonuses to rarity/quantity. Any quantity bonus from a Map is multiplicative with all other modifiers.
Effect on Experience
See experience in party for more information.
Effect on Maps Dropping
The chance of a Map dropping is not affected by party size. They are affected by the quantity and rarity bonuses provided by the Map they are dropped in side of. Unique maps are dropped as part of the normal Item drop system and not governed by the same rules as other dropped maps so they do get the party bonus. However, item quantity still allows players to potentially have better chances of getting maps because iiq increases the quantity of items dropped by monsters killed.
Effect on flasks
Party members of a character landing the killing blow on an enemy will gain Flask charges if they are near by. This is not true for all +Life and +Mana gained "when you deal a killing blow" modifiers; only a character actually landing a killing blow will trigger those effects.
It is unclear if flasks still have a +75% charge recovery bonus for each party member after the first. If this mechanic is still in the game, members of a full party would gain 375% more flask charges from killing monsters.
- Mark_GGG (March 9, 2014). "Flask charges in party". Official Path of Exile Forums. Retrieved March 18, 2014.