I often tend to say something like
Who all is coming to the movies?
And my friends correct me that I should be saying
Who all are coming to the movies?
So which one is correct?
In (American) dialects that use this variant, "who all" is actually a pronoun in its own right; it's sometimes written "who-all". (Bear in mind that this is an extremely informal usage, and so it's rarely if ever written down at all by the people who actually use it - only by ethnographers and linguists who are studying the dialect, and novelists trying to add a little local color.) The region where it's used overlaps, but isn't exactly contiguous with, the region(s) where "you all" (or "y'all") is common.
In usage, just as "you all" can be treated as a substitute for "you", "who all" takes the place of "who" - so I think you'll find that most American speakers (who would use this construction) would ask Who all is coming to the movies?
Both are incorrect.
MT_Head's answer sounds right to me when it comes to southern US English, but in Indian English, the situation is a little different - "who all are" is the correct plurality for the verb.
I don't think it's correct to categorize the Indian English version of "who all" as a pronoun. At minimum, there is no analogy to "you all", since that isn't a lexical item in any variety of Indian English I've heard (though the sequence "you all" obviously still exists in contexts like "Are you all going to the movies?"). I don't have the linguistic vocabulary to accurately describe what's going on in "who all are going to the movies", but the "all" is sort of a "modifier" here; "who all" doesn't strike me as a discrete lexical item.
Those of you unfamiliar with Indian English may be surprised to learn that "all" can also modify other interrogatives, which is something that I don't believe is a feature of southern US English. For example:
I'm not sure whether I've personally heard "when all", "how all", or "why all", but a quick bit of googling for constructions like "why all are you" does reveal a fair bit of Indian English that uses these other interrogatives with "all" as a modifier.
I think that interpreting "who all" as if it were dominated by the word all instead of by the word who misunderstands how the word is used in regional U.S. parlance. Presented with the question
most U.S. English speakers, I think, would choose is over are as the verb to use. The appearance of all in the formulation might seem to alter the logic of the question, but it doesn't. The question
really amounts to asking
With or without the all, most U.S. English speakers who are familiar with the "who all" phrasing will understand the question to be asking idiomatically "Who is doing X?"
Similarly, a parent using the dominant form of U.S. English with regard to this detail of language might ask a crowd of kids at a birthday party
while a parent using the "who all" form might ask
Neither parent is asking which one child wants ice cream; it would be absurd to suppose that only one child among many at a birthday party would express a desire for ice cream. And yet both parents are extremely likely to frame the question as a singular: "Who [all] wants..."
Because I grew up in a part of the United States (southeast Texas) where "who all is..." was very common in informal speech but where "who is..." was essentially the only accepted form in writing and in formal speech, both wordings sound entirely natural to me.
Indeed, in that part of the country, a construction of the form "...are coming to the movies?" would sound right only if it began, not with "Who" but with "Which":
I suppose that the shift in this case happens because the "Which of you..." wording (again in U.S. idiomatic speech) implies "Which ones of you..." although "Which of you is..." is also a common form when the intended sense is the singular "Which one of you is..."
As far as I know, there is no idiomatic "which all" wording corresponding to "who all."
Yes the usage of 'who all' and 'you all' seem to be more of a direct translation from one's vernacular to English as far as Indian English is concerned. I state so as far as from what I have read so far I have never come across such a usage by any good authors. So it does seem to me that it is not a part of the formal English that is spoken or written.