All members of such an insurer shall agree in writing to pay their pro rata share to the insurer of the necessary expense and loss sustained by any member thereof during the time for which their respective policies are in force. They shall also, at the time of effecting the insurance, pay the insurer such percentage of the estimated cost and such other charges as are required by law or the insurer’s rules and by-laws.
(Amended by Stats. 1939, Ch. 931.)