You can amend the complaint to add the driver as a defendant.  File the amended complaint with the court with a proof of service and have it served on all defendants.  When the amended complaint has been served on all defendants, file a certificate of service with the court.  The certificate of service filed with each proof of service shows the court that the amended complaint was served.  In CA, the certificate of service has to be filed with the court within 60 days of the date the amended complaint was filed with the court.