And that's pretty much all I can remember about it, unfortunately. There was, I'm fairly certain, a human protagonist, and s/he stumbles upon a group of faeries/elves/something similar masquerading as homeless human teens in a large city--I believe they were living together (squatting) in a brick building, and they had some way to access an alternate, magical world. Possibly something terrible was happening in the magical world which required the humans' help.
I read it as a kid, so it was probably published in the 1980s.