The party system is the way you may group up with other players to clear dungeons and share experience. The party system allows players to join a party looking for players as long as the window is above the head of the party leader by clicking it with the pointer (alt or esc). To form a party, click the "form party" button in the social tab when you open the menu (alt or esc). Once you click it, a box will appear and you will be able to write a message which can be seen in the party flag. To fight dungeons with your party, simply go into the same dungeon entrance on the same difficulty setting. You may still use partners if you choose so.
You and your party will get their own separate items and experience from a kill, but the person who struck the killing blow will receive a larger amount of experience and has a chance at extra items (such as a lucky box).
The party sizes goes up to five members and the members are shown in the middle left side of the screen UI. Note that the dungeon difficulty depends in the party size and not levels. The party members can chat using the party tab in the chat. The chat shortcut for the party tab is "/p".