According to the San Francisco Tenants Union, Homes Not Jails:
"was formed in 1992 to advocate for the use of vacant and abandoned housing for people who are are homeless. With people literally dying on the sidewalks in front of vacant buildings, housing advocates, homeless advocates, and people who were homeless came together to find ways to utilize vacant buildings." - website [link broken 3/31/19]
HNJ works on two fronts: they advocate policy changes, and some of their members might squat and open up vacant buildings.