The leader of a phony car-crash ring pleaded guilty to federal mail fraud charges, along with another man who has agreed to cooperate in a continuing investigation aimed at doctors and lawyers. Harold Betts, 33, presently an inmate at Boron Federal Prison, pleaded guilty to four counts of mail fraud in what Assistant U.S. Atty. Ronald Nessim said may have been hundreds of bogus accidents that led to collection of millions of dollars in insurance payments. Lynn Collier, 26, of Los Angeles, pleaded guilty to two counts of mail fraud arising from the same case. Nessim told U.S. District Judge Edward Rafeedie that Collier has agreed to cooperate in the continuing investigation of the ring, but Betts has refused to assist authorities. Gregory Hill, 31, of Los Angeles, pleaded guilty earlier to two counts of mail fraud in the case and awaits sentencing, while two other suspected members of the ring await trial.