The trial court did not abuse its discretion under CCP 389 in concluding that residents of a mobile home park were necessary parties to the park owner’s mandamus proceeding against the Santa Cruz County’s rent control board.  Before that board, the residents had opposed the park owner’s application for a rent increase, even presenting expert testimony regarding the owner’s claimed costs.  The trial court correctly sustained the county’s demurrer with leave to amend.  But when the park owner chose not to amend, the trial court erred in automatically dismissing the suit without first considering under CCP 389(b) whether the residents could be made parties and, if not, whether the matter should be dismissed or proceed in their absence.