Who is “they”? This is the most critical question, as it seeks to unmask the unspecified. Can the speaker be explicit? Does the author feel comfortable attaching more detail to the group being labeled? In large organizations, sometimes asking for clarity on who “they” is forces someone to realize that they don’t actually know who instituted a policy, or aren’t really sure where guidance came from – it was just accepted as coming from “them” and accepted as real.
And to quickly address the obvious: it’s not technically grammatically correct to frame the question as “who is they”. By convention, “who are they” is correct. But sometimes “they” can be a proxy for one person who may or may not have been speaking on behalf of a group.
“They said no parking here.”
“Who is they?”
“Some lady walking down the other sidewalk.”
“Does the lady walking down the sidewalk have some kind of authority? Or are we just assuming she is right and deputized to speak on behalf of enforcement.”
“I’m not sure…maybe we can park here.”

