§ 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.