They don't want anybody to freak out that they were abducted and sent to another world where they were used as batteries for mirror creatures. Those creatures need them to stay outside. Inside the mirror world they are under the belief they always lived in the world, basically brainwashing. The gang is not sure they will remember don't want the civilians to live in fear of it happening again. So while the civilians are knocked out in the real world after being rescued. They set up part decorations around the small town to pretend they had a party and substance(gas or drugs) to trick the civilians into think they were spiked.
Mne is basically this. A world that was an gray endless void, but kidnapping a person and attaching a small glass to their person turns the world into a reflection of the real world, except the color is mixed up. Sky is green, grass is red.
buddy, take it from someone who almost exclusively has written drug addicted protagonists--using cocaine and alcohol is ALWAYS the way to go. Shared dream bullshit, people can question that. But a drug-fueled party full of gore and sex? no one thinks twice about it.