During the scene in the final episode, people start getting mentally possessed and leaving the party and walking toward the mind flayer.
You even see a wife ask her husband where he's going when he gets possessed. Then a little kid asks his friend where he's going after he gets possessed.
Why didn't everyone get possessed and start walking? Why was it only a select few?
Those were people who had previously been abducted and taken to the factory to be infected. First it was Billy and the lifeguard girl working alone, then as their ranks grew I assume the already converted were helping to kidnap or trick others into going there. It wasn't random individuals being spontaneously possessed at the celebration. All the Mind Flayer's minions were getting a recall command, and stopped whatever they were doing to report back to base.
It makes no sense how these people are selected, especially given the fact that the only person previously flayed (season 2) was Will Byers. Why, at its full power, was Will the only victim in season 2, yet all these very specific people - and not others within the same vicinity - were flayed in season 3? Surely, the mind flayer could have possessed more people in season 2, if only a piece of it left behind in season 3 possessed hundreds of people. And surely, if it got a man, why didn't it get his wife and kids? It puzzles me.
The only thing I can guess at is that there's some element of darkness or evil in these people, as we've seen in Billy, that opened them or made them more susceptible to being flayed over other people. Maybe it gravitates towards damaged souls? We can only guess.
Each of the flayed was either tricked into going to the factory, or abducted and brought there by force, just like Billy did to lifeguard girl and later both of them did to her parents. As more and more people were assimilated they helped deliver more victims to add to their ranks. But every single Hawkins resident who responded to the Mind Flayer's call and walked away like zombies had previously been to the factory and infected by the monster. That's why some and not others.