Parties in maps
Entry. One player opens a map at the device, and the rest use the device afterwards to join the same instance. Six entries in total. The opener pays the gold fee; others pay nothing (The map device). Everyone shares the opener's Focus (Atlas Focus).
Monster scaling. Monsters in an instance gain more life for each extra player present when the monster is first engaged. With six players, they have about three and a half times the life. Their damage does not change, so difficulty comes from the longer fight and from rare and magic modifiers (Monster modifiers).
Loot scaling. More items are rolled by the same factor, then drawn among eligible members (Party experience and loot). Per player per hour, the loot stays close to solo play. Map modifiers' quantity bonus applies once for the party, not for each player (Map modifiers).
Bosses. Map, act, pinnacle and Voruun Hale bosses also gain life per extra player, or follow their own encounter tables.
Locked at the start. The number counted for a monster is fixed at first engagement, so a player cannot join, leave and cheat the scaling.
Credit. Faction Standing needs 5% of a mini boss's damage; Seals need 10% damage or equivalent healing. Healing only counts when a party member really took the damage (Standing and Tokens, Pinnacles and Writs).
Classes working together. Templars heal and shield the party (Ascendancies); knights draw monsters; casters clear packs; flask builds keep going while the party killing fast supplies charges.
Dying. You remain in the party and can walk back in through the portal while entries remain (The death rule).
Anti-abuse. Low-level leeches, AFK passengers and paid carries gain little. Seals and Writs are character-bound.