1.  Availability.  Except as provided in subsection 2 or 4, insurance purchased under section 18051 must be made available to the following persons:  

2.  Exception.  The board may, by rule, provide standards for inclusion or exclusion of employees on the basis of nature and type of employment or conditions. No employee or group of employees may be excluded solely on the basis of the hazardous nature of employment.  

3.  Employment with 2 employers.  Except as provided in section 18056, subsection 3, paragraph D and section 18656, subsection 3, paragraph D, a participant to whom coverage is available under subsection 1 or section 18655, subsection 1 may participate through employment with 2 or more employers.  

4.  Retirees who return to service.  A retiree who is insured pursuant to section 18061, subsection 2 or 18661, subsection 2 and who returns to employment in a position in which the person would be eligible for coverage under subsection 1 is eligible for coverage under subsection 1 only if that person pays the cost of the coverage. This limitation applies regardless of whether similarly employed persons who are not retirees would be required to pay the cost. The board shall establish a method by which such persons may pay the costs of insurance coverage elected under this subsection, which may include basic and supplemental insurance.