ejcasey and SaraGalesa are correct. Did you guys report it?
"Whom did you invite?" is grammatically correct. It is also not how most people talk, so probably "Who did you invite?" should also be accepted, because in the real world, in North America, anyway, that it what people would say.
"Who did you invite?", the suggested Duo answer, is not grammatically correct.
Well here in America I have never heard it asked any other way. WHO DID YOU INVITE? WHOM WOULD GET YOU LOOKED AT FUNNY.
Maybe, but Duo also accepts the grammatically correct, "Whom did you invite?"
It's one thing to accept an incorrect response due to widespread usage, it's another thing to promote it.
"Whom did you invite?" is correct. At least that's the way this Mid-westerner speaks English.
"whom" not "who" - it is the object of the verb invite. "Who did you invite" is wrong even if it is colloquial.
While you are correct that grammatically it should be "whom," nevertheless, most people (that I know, anyway) would say "Who did you invite?"
Just like ain't used to be incorrect, now it is correct because the majority of the population uses it. Whom is the same way, it sounds old English
Splitting hairs here, but isn't it "Whom," not "Who"? As in, "Whom" is the object of "did you invite?"
It depends if one wants to learn English or "talk like an American".
American journalists and authors have largely abandoned grammar in pursuit of "the conversational tone" and to avoid the dreaded "stuffiness".