Who of you?

[BobK]

Speaking as another 'crepuscular' person (OK, it doesn't mean that, don't use it, but you get the point) I would use both:

Which of you will help me?
Who of you is supposed to help me [because it's your turn on the agreed rota]?

Raymott's tongue-in-cheek suggestion points at the reason for this choice. 'Who among you...?' is a familiar biblical phrase.

b

[petit_minou]

Haha Sorry, I should have said a younger native speaker would not use that phrase, not all native speakers. Clearly that is incorrect although older native speakers would probably not say it. There is nothing grammatically incorrect about it.

I am far from the grave (27) but still have a lot to learn

[Vidor]

"Who here can help me?" is also acceptable. Or simply "Who can help me?" Although like "Which of you can help me?" it carries an air of expectation with it.

"Who of you" I'm pretty sure is not used in modern idiomatic English.