"Who is" is typical English usage, even in a lot of cases when the expected answer is plural. "Who are in the back?" sounds weird to my (native-English) ears. "Who is going to the party tonight?" (even if you expect the answer to be many people) and "Who is coming across the field?" (even if you're pointing at a whole group of people when you ask it) are normal, and would sound wrong to me with "who are".
On the other hand, if the subject is named by an explicit plural, then it seems "who are" is more normal:
"Who are those people?"
"Who are the teachers?"
and so on. Here's a pretty good discussion of this subject:
Means something different to "who is in the back". "who are they at the back" is asking about those particular people - who happen to be at the back. "who is in the back" is asking which people (if any) are at the back.