Filling in new players on everything that happened in the campaign before they joined is something that happens at best loosely and in a perforated fashion (i.e. full of holes).

This is often sensible, because if their character is also new to the ongoing adventure, then the character won't know much about the previous events either. And even better, it means that there can be fun revelations where the new person says, "Wait, what?" and everyone else goes, "Oh yeah, that happened, we all knew that."

Transcript

[SFX]: Kerrunch!
Luke: Whoa. I never knew... any of this. Is that all true?
Chewbacca: ... From a certain point of view.
C-3PO: Which is to say, yeah, it all happened before you joined the game.
Luke: So much betrayal.
C-3PO: Yeah, that was kind of Anakin in a nutshell.
R2-D2: Like that little girl in your Robot Utopia campaign. Who just happened to have Master level Hacking on her skill list. She totally made out like she was on our side.
C-3PO: Hey, turning the fences back on was sensible at the time. Any human would have done the same.
R2-D2: She was a better hacker than any of us robots! How is that fair?
C-3PO: You were robot dinosaurs.
R2-D2: She betrayed us!
C-3PO: She still liked the veggie-sauruses!