That "joiner" created a dangerous situation for everyone behind him. The OP already mentioned the wheel overlap. Someone in a paceline cannot slow down without signaling those behind. I suspect that this one person, who joined the paceline mid-ride, rotated to the lead, then slowed down has probably never ridden in a fast paceline before. That's downright dangerous, for everyone!

We do not know how much verbal communication there was between him and the others. If he had an opportunity to ask if it was okay to latch on, that he was inexperienced in paceline riding, but would it be okay to join in anyway . . . then the others would be aware of his limits/experience, and would keep a close eye on him.

Getting passed, or passing others while riding solo is a whole different ballgame than riding in a close paceline and have very little to do with each other, besides being on bicycles going in the same direction.