You can enter the Cross-Server Dungeon Lobby by use of a Cross-Server Transit Shrine.
One Cross-Server Transit Shrine can be located in the North-East corner of Victory Plaza within Hakain's Crossing. This Transit Shrine will allow you to enter any of the Cross-Server Dungeon Lobbies.
Additional Cross-Server Transit Shrines are located outside of their respective dungeon entrances. Entering from one of these locations will place you in the appropriate Dungeon Lobby.
Cross-Server Transit Shrines are also available within the Dungeon Lobby when you need to travel between the other dungeon entrances.
An Exit Shrine will allow you to leave the area to travel outside a specified dungeon, or back to Hakain's Crossing.