Welcome to the Pig Sty guide at TV Tome. 
The guys live in apartment 15C of building 20 on 89th and Amsterdam (the streets are mentioned in "Erin Go Barf" when Iowa takes a cab home). Iowa and Johnny share one room, Randy and P.J. another, and Cal lives in the walk-in-closet. They have a shrine to Bob Elison, who lived there for a year until he got married. Apparently they have a spare bedroom to use in case one of them gets lucky. (This is from "Beauty & the Beasts," but doesn't make any sense since otherwise they probably would have put the fifth person in this room, instead of moving Cal into the closet. In "The Maltese Falcone" the spare bedroom is mentioned again.) In "Tess Makes the Man" it is implied that the guys have more than one bathroom. Randy, Johnny, and P.J. seem to know each other from college, and Iowa is Randy's brother's girlfriend's cousin. It's not clear how the know Cal, or why they even lived with him to begin with, since he is a complete pig. In "Beau