A party is automatically created when a player invites another to a party.
To invite someone, simply right click his portrait and click "Invite Player".
Simply right click your own portrait and click "Leave Party".
You must be invited by the party's leader.
The player who initiates the party will be the party leader. He or she can kick players, reset dungeons , set different loot rules and promote another player to leader. Only one player can be the leader.
If a player gets disconnected from the game, he or she will be shown in the party interface with no health or mana. After reconnecting they'll be teleported to outside the dungeon, but can rejoin his or her party normally.