Only players in the same area as you see your party in their party window. You can invite other players to join by either left clicking on them and selecting "Join Party" in the character menu, or by using the "/invite" command in the chat bar.
To enter a dungeon as a party, all party members are required to be in the same waiting zone. Once all members are present, the leader of the party can enter the dungeon area.
Once in a dungeon, party members should aid each other in defeating monsters, buffing one another and in the case of the Priest, healing his fellow party members.
When a player picks up any item that’s of Uncommon rarity or higher, a die is rolled automatically for each player. The player with the highest roll wins that item.