Section 91. Groups for accident prevention

For any group established under
  the provisions of section ninety membership in the group of any employer
  otherwise entitled to  be  admitted  thereto  may  be  conditional  upon
  acceptance  and maintenance of special rules as to administration and as
  to accident prevention and medical care of employees. Such limitation of
  membership in the group may be established  only  upon  proper  evidence
  that a majority of the members of the group have approved such rules and
  only  when  such  rules  have  been  approved  by  the  commissioners as
  sufficient to  constitute  a  proper  basis  of  differentiation  as  to
  membership in the group.