The Expedition System (also known as 'Raids) could be accessed from the Party menu when it was first added into the game during Open BETA. While, the player could access the "Expedition" tab, no dungeons had been implemented yet and as a result, the player could not actually create or partake in an Expedition. They were officially added in conjunction with the "Requiem" content update.
Expeditions are, as described by the official article regarding the subject, an almost endless onslaught of enemies designed to challange either one person, or any number of people up to a maximum of eight.
The "Requiem" update introduced players to the first Expedition dungeon, "The Altar". According to the staff, there will be more dungeons in future.