The Invitation box is obtained during the Missing, Presumed Death quest after killing all wights. The box is used to teleport to the Empyrean Citadel. After the quest it also provides a quick teleport to Brother Samwell, useful for accessing the Odd Old Man and two beacons from the All Fired Up minigame. To do this, the player must activate the box once to teleport to the Empyrean Citadel and then once again after arriving there to teleport to Brother Samwell.
If you lose your Invitation box you can reclaim it from Brother Samwell or from the box in place of him after Kindred Spirits.
It is possible to have more than one invitation box by reclaiming it while the invitation box is in the bank.