Surely either 'Who is with you?' or 'Who are you with?' should be accepted?
"Who are you with" should definitely be accepted (in fact, it was accepted when I entered it). "Who is with you" would translate to «Кто с тобой» if I'm not mistaken and shouldn't be accepted.
Seems it's already accepted, but it's supposed to be the actual example and not the other way around. Is "With who are you?" grammatically sound to begin with?
I noticed this pattern on the iOS platform, where it seems a little harder to give feedback about problematic questions/answers. I'm relatively certain that "Whom are you with" would be the correct English grammar. My sense as a native (American) English speaker, is that informally in common speech, the with who/with whom and for who/for whom etc distinction in English is often ignored...
While "whom" is correct. It's very formal. Certainly in British English using "whom" in anything but a set phrase in conversation would appear quite strange.
So, what does this actually mean? ‘Who are you seeing/dating?’? ‘Whose side are you on?’?
It can mean either of those, given a proper context. But there may be no metaphoric meaning at all, like in the following dialogue: "I’m at a party now." "Who are you with?" "With some friends."
Now if we were being picky, this should be whom, not who. It's incorrect English, but everyone says it anyway.
It sounds unnatural. I think you would only use that word order for emphasis, not as a regular question.
For example: "I am with President Obama." "You are with whom???" [to express surprise (or outrage)]
I wrote "Who are you with?", the exact correct solution, and it was wrong...?