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.