Can groups of people "remember" something that didn't happen?
WE KNOW THAT IT'S RELATIVELY EASY to implant memories. We also know that once a memory has been implanted the process continues and there are studies on what's called updating. Take the case of false news about the Iraq War early on which was discovered to be false; in one study if you were in Germany or Australia you were likely to update the information, but if you were American you weren't as likely to update the information. So for some people, even if you were told a fact was wrong, you would still remember the previous incorrect fact. This suggests that memory is schema consistent, so if something fits into the way you think things should be it, you don't easily revise the memory once it's been formed.
A schema is sort your organized representation of the world. When something fits into your view of the world, you're primed to absorb that information. If it's mildly inconsistent, it sort of stands and becomes memorable. But if it's too inconsistent then it slips away. In terms of groups talking together, studies suggest that false memories are more likely to arise in a group discussion than individually because there is more chance of somebody offering a false memory which can then be implanted. However, if somebody in the group says "no that's not true", that will mitigate the influence. But for groups with a strong shared interest, like Trump supporters, they're less likely to dispute one another and therefore it's less likely for their memories to be updated.
From all the evidence, memory is not like a tape recorder. There's not really a "truth" to memory. What Frederic Bartlett said is that memory is a continuous reconstruction. And what guides your reconstruction? Your view of the world. Your current view of the world, your current attitudes, allow you to reconstruct your own past to be consistent with your present self. We're constantly reshaping our memory to essentially reinforce our present attitudes.
Also, every time we remember things, we're selective. Our memories are to a large extent determined by our interactions with others, and by others I mean other people, media, and other external factors. Our memory is not only our own, but also that of all we interact with.