Right-click on the barrier itself and choose to "Inspect the Barrier".
Find the a gift mimic at ( 52, 358, -129 ) /waypoint 52, 358, -129, right-click it and choose to "Inspect the Chest".
Get the chapel door key from the rotting dancer at ( -73, 364, -128 ) /waypoint -73, 364, -128. (You will need to clear the trash in this room to make the wedding cake collapse, showing the rotting dancer).
Go back to the barrier and use the following sequence of emotes (these are group updates):
The barrier disappears. Click on the door to unlock it, to allow entry to the chapel.
Speak to the Ghostly Witness at ( 3, 353, -46 ) /waypoint 3, 353, -46 to complete the quest.