"Who all...?" is incorrect English. It is used in the South, in the United States, and it is dialectical. Southerners also contract "You all" into "Y'all": "Y'all come back now, hear?" That is Southern dialect for "All of you are welcome to visit again any time." Standard English is simply "Who ...?" (or "Which of ...?") "Who came?" "Who was here?" "Which of you were here?" "Which of you came?"

Can we ask formally. Who all came here?" as the Interrogative of "You all came here." ? It is the short, informal way to ask the question and might not be understood or considered standard in some parts of the world. More formal would be the following: All of you came here? or, Did all of you come here? or simply, Who came here? "Who all came here?" and "Who came here?" really mean the same since "who" is indefinite and can indicate plural or singular. It might not be understood because "Who all came here?" can also mean "Who among you came here?" This would not include those among you who were "already" here. That is how I understand your question. Denis :)