Content warning: gaslighting, arguably.
You'll need 3 to 5 players, and paper, pens and an envelope for everyone.
Give each player ten minutes to invent a memory of something you all did together. A party you went to, a movie you watched, a meal you ate. Something communal, something plausible but definitely untrue, something that "happened" within the last five years.
It should be a nice memory. Something low-key. Something charming. Something that could well have happened, something you kinda wish was true, something that would be true in a very slightly better version of your life.
Don't tell anyone else your memory - just write down a one-sentence summary on a strip of paper. Copy that exact sentence out once for each player. That Boxing Day when we couldn't find an open pub so we drank corner-shop beer on a bench near the river, sleeves pulled down over our fingers to keep them warm, and we saw three foxes run along the middle of the empty pavement. That night when we went out dancing and it snowed while we were inside, and we came out at two and everything was white in the streetlamps, and we were drunk and it didn't feel cold and we threw snowballs in each other's faces in the park and fell over a lot. That autumn morning when it was suddenly hot, and we went to the park and there was a power cut at the cafe so they were giving out their ice-cream free before it melted, and someone thought they were getting mint but it turned out to be pistachio and they were so sad, who was that anyway? That boat trip at sunset. That picnic where a squirrel ran up and stole a whole cupcake, what the hell.
Now, fold up your pieces of paper and hand them around the table. Everyone should have one copy of each sentence. Don't read them: just put them in your envelope. Seal your envelope. Write MEMORIES on the front. Keep it safe.
Over the next few years, try to seed your story into your friends' memories. Mention it sidelong. Tell the story to an acquaintance within your friends' hearing, as if of course they know all about it. If you really want to put the work in maybe take a picture that could fit the story and post it among a pile of others, "oh I came across these old photos haha remember the past". Don't belabour the point, though.
Then, some time - three or five or ten years from now - say to one of your friends: oh, remember when such-and-such...? And if they say yes, if they help you tell the story, if they add their own details, then you win. Take out the envelope and confront them with their false memory, if you like. Or just let it rest. Either way.