The invitation for all of these characters can be found on their bodies after being killed. Thanos' body comes out of the bottom of the elevator, not the top.

After she is killed, talk to Grinmaw. He will spit up her invitation to the top of his tank. Simply climb up the ladder and pick it up.

Talk to the ghosts in Tequila's Stage (where she was performing). One of them will give you her invitation.

Open the safe in the chapel before Sixpence does. You can do this using Greyson's power. Inside the safe will be his invitation.

Pick up the Hungry Charm. Then, after Aurum has left to meet with Thanos, talk to the male ghost in the lower righthand corner of the room with the statue and ghosts. He will have pickpocketed the invitation from Aurum and will trade it "something gruesome", aka the charm.