Hmm, most sources consider it wrong and my own native-speaker intuition is that it is very, very odd sounding.
The only exception is when using the verb to be as a copula, linking "who" with a plural noun or pronoun.
Who are they?
Who are those people?
Who are your friends?
In those cases, you could argue that the plural noun or pronoun is syntactically the subject and "who" is only at the beginning because of wh-fronting.
Who is eating? (Not "Who are eating?")
Who is in the room? (Not "Who are in the room?")
Who sleeps together? (Not "Who sleep together?")
"They hate chocolate"
"Who hates chocolate?" (Not "Who hate chocolate?")
It feels a bit awkward whenever you have "each other" straight after it though.
Who hates each other ... doesn't feel quite right.
The relative pronoun "who" can go with plural verbs:
All the people who are coming to the party ...
Possibly, "Who are playing music?" is grammatical in some dialects of English, but I don't think I've ever heard a native speaker say this.
Both are grammatical andcan be used, but the more informal the register the less likely that "are" will be used. http://ell.stackexchange.com/questions/52038/is-who-singular-or-plural