§ 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.