Employees eligible for participation in the fund shall include all employees who are employed by the city on a permanent basis. The following employees, however, shall not be eligible for participation in the fund:

(1) Appointive members of administrative boards and commissions, except employees of such boards and commissions;

(2) Individuals employed under contract for a definite period or for the performance of a particular or special service;

(3) Employees serving on a part-time basis of less than one-half time;

(4) Policemen and firemen covered by a policemen's pension and relief fund or firemen's pension and relief fund;

(5) Employees who are paid in part by the state, county or other governmental agency, and only in part by the city;

(6) Employees who are past sixty years of age on the effective date of the fund; and

(7) Employees who are hired after the effective date of the fund and who were past sixty years of age at the time they were so employed. Effective January 1, 1987, a city may disregard this exception.

In case of doubt, the board of trustees of the fund may make determination as to any individual's eligibility to become a member of the fund.

All employees eligible for participation at the effective date of the fund shall become members of the fund, unless they file a written election not to become a member within thirty days after the effective date of the fund.

Effective January 1, 1987, a city may provide that employees who did not participate in the fund when first eligible, or who were not permitted to join the fund when they were first hired due to the prior age sixty limitation, may now participate. Such members may purchase prior service by paying into the fund the employee contributions they would have contributed had they been in the fund plus interest at the rate of six percent annually. Members shall be given two years to pay these contributions.