Five characters, one party
Bring your own characters along as a party, and team up with other players.
Your own party
Your account holds up to five characters. When you enter the world, choose a leader, then pick the others in Who comes along and confirm. Only the leader walks; the members you brought follow it and fight beside it.
Playing with other players
Invite another player to your party to fight in the same battles. Quest kills by your party in a shared fight count for everyone who carries the quest, and quests can be shared to the whole party. A party enters a dungeon together.
Build a balanced party
A Warrior in front covers the back row, a Priest keeps everyone standing, and a Mage or Ranger deals the damage. See The formation for how rows and lanes work.