Hi, I am aware that this is a very fundamental grammar question, please bear with me.
My question is pretty straightforward, it's just what the title says; is it grammatically incorrect to say "Who are coming?" or "Who are playing in that concert?"?
Now, I know a lot of people would say that it's wrong, and people usually say "Who's coming?", but what if the person's asking "Who are coming?" because he knows it's going to be people, not just one person?
I've googled this, and the answers on the internet did not explain this very well, that's why I'm asking here. Because we do say "Who are these people?" or "Who are they?", it's hard to just predicate that "who" is singular.
I will really really appreciate those who answer my question. It's been bugging me the whole day.

Thanks in advance!