SAN RAFAEL—A small nonprofit last year formed an unusual support group for troubled Marin County homeowners. The group has helped members file dozens of lawsuits against mortgage companies, in a sometimes-controversial tactic aimed at staving off foreclosures.

The group's members say they have had some success in buying time or bringing the companies to the negotiating table. Others say they would have been kicked out of their homes if not for the group, called Families Fighting Foreclosure. The lawsuits prevent the mortgage...