Recently I read on a language website about how a student was confused about when to use do after who in questions, as someone had told him we can't use do/does/did after who. I don't know who that 'someone' was, but they had obviously got a bit confused themselves. Yes, there are times when we don't use do/does/did after who (and some other question words), but it is really the exception rather than the rule.
Read about using auxiliary do/does/did in questions, and do a couple of exercises.
