you are waiting for some friends to pick you up, someone knocks on the door and you ask "siz misiniz". Is that a possible interpretation?
So in English, you have to always have a subject. In this sentence, there is a dummy subject which doesn't really have any meaning unless there is a context. "it" can mean anything you want in this sentence.
Would the question be correct/complete if we just ask 'misiniz'?