who have you been dancing all night with you? Correct: "Who have you been dancing with all night? " You got the "who" part right, you just added an extra word:) Since you already said "you" at the beginning of the sentence, you don't have to say it again at the end, which is nice. Examples of who questions: Who are you going with? (only one direct noun needed) Who is that girl over there? Who are those boys playing soccer?

I believe a gramatically correct way would be ... With who have you been dancing all night? You don't say (you) twice in the sentence though since you've already stated who you're addressing.

Layah gave a great answer, but I just wanted to add one thing. Grammatically, you can't end a sentence with a preposition (such as "with"), but in normal conversation, we all do it. For example, "Who have you been dancing with?" is natural, but technically, the grammar is wrong.