Mass. Gen. Laws ch. 40N, § 7
(b) Every employee who immediately prior to being transferred to the commission by this section is a member of the municipal contributory retirement system established under chapter thirty-two, or predecessor retirement systems thereto, shall continue to be a member thereof and subject to the laws applicable thereto. All other employees of the commission shall be required to become members of the municipal contributory retirement system in the same manner and subject to the same laws, rules and regulations as persons entering the employ of the municipality. The commission shall deduct from the wages of its employees and pay over to the municipal retirement board such sums as the municipality would deduct and pay over if such person were an employee of the municipality.
The commission shall reimburse the municipality for its proportionate share of any amounts expended by the municipality under the provisions of chapter thirty-two for retirement allowances to or on account of employees of the commission.