Group life insurance may be issued covering the members of 1 or more companies, batteries, troops or other units of the national guard of any state, written under a policy issued to the commanding general of the national guard who shall be deemed to be the employer for the purposes of this chapter, the premium on which is to be paid by the members of such units for the benefit of persons other than the employer: Provided, however, That when the benefits of the policy are offered to all eligible members of a unit of the national guard, not less than 75% of the members of such a unit must be so insured.