West Marin housing advocates are joining forces to promote a $20 billion Bay Area affordable housing bond that will be on the ballot in November. The measure would allocate about $700 million to Marin County. West Marin Community Services, the San Geronimo Valley Community Center and the community land trusts in Point Reyes Station, Bolinas and the San Geronimo Valley are now part of a countywide coalition calling itself Housing for All Marin. The groups…