Group life insurance may be issued covering the members of any labor union, or state associations of teachers or postal clerks written under a policy issued to such union or association which shall be deemed to be the employer for the purposes of this chapter, the premium on which is to be paid by the union or association or by the union or association and its members jointly, and insuring only all of its members for amounts of insurance based upon some plan which will preclude individual selection, for the benefit of persons other than the union or association or its officials: Provided, however, That when the premium is to be paid by the union or association and its members jointly and the benefits are offered to all eligible members, not less than 75% of such members may be so insured: Provided further, That when members apply and pay for additional amounts of insurance, a smaller percentage of members may be insured for such additional amounts if they pass satisfactory medical examination.