Somerville Mayor Katjana Ballantyne announced that the city will be joining a coalition of cities experimenting with a guaranteed basic income by giving 200 families at risk of homelessness $750 a month for a year. “We’re trying to use any tool possible to help these families become stable,” the mayor said in an interview. Her office worked with the public schools and local social service agencies to identify the residents in need, part of the city’s comprehensive effort to offer financial literacy, rental vouchers, and other measures to keep vulnerable Somerville residents in their homes.