You'll have to collect all of the guest invites from the game. Because of the nature of the game, spoilers follow.
*** Spoiler - click to reveal ***From left to right in the menu under progress:
Reginald - You'll find this on his body in the chapel. He's the first victim you save, but you won't be able to get it until you return here later in the game.
Trinity & - Clay - Both of these invitations are on their bodies. You'll have to wait for them to die, and then search them. Clay's is easily accessible, but you'll have to have saved Trinity, listened to the staff punch in the door key code, and then use that keycode to open the door to the room in which she dies.
Willow - This is the woman who hangs herself. Once you have her power and have collected the lighter from the safe in the theater, return to her secret room behind the skull door, light the candle in the room, and talk to the fish. After this, the invite will appear on top of its tank.
Tequilla - After she dies, light the candle in the piano room where the glass shattered. One of the two ghosts here will have it.
Redd - This invitation will be found on the victim's body. You'll have to wait for both people in this room to die and for the masks to fade away before you can access this invitation.
Grayson - Once you've saved Grayson, you'll have the ability to pick locks. When you finally have the chance to return to the chapel where Reginald was killed, get to the safe that he opens before he does. If you unlock that safe (the one from which the blank bullet comes), and open it before Reginald, you'll get this invitation.
Aurum - This invitation is found by trading a full sated hunger charm (which you'll figure out how to get through story) to a ghost in the bar. He will walk through the bar, which will have a slot machine, and the ghost will pickpocket it.
Thanos - This invitation is on Thanos' body. You'll have to enter the basement by one of the ladders and loop back around to the bottom of the elevator that Thanos dies in. Eventually, he'll wheel himself out of the elevator on fire, and you'll be able to collect the invitation.