"Hvem er ved at gå?" means both "Who is leaving right know?" and "Who is about to leave?". To emphasise that you mean the latter, you would say "Hvem skal til at gå?" or, as you suggest, "Hvem er lige ved at gå" (note the spelling of "lide". Saying "Hvem er ved lige at gå" is strange, although it is not totally incrorect.
In the comments section people always said that "at gå" specifically refers to the act of walking, but here it is translated with "to leave". But leaving cannot be realized walking only, you can also leave by car or by plane.
So wouldn't this sentence be better as "Hvem er ved at tage afsted"?