Mass. Gen. Laws ch. 32 § 28K
Any employee of the commonwealth or its political subdivisions who is a representative of an employee organization, which has included in its membership employees of the commonwealth or its political subdivisions shall, while on a full-time or part-time leave of absence for the purpose of acting as a representative of said employee organization, be considered on leave of absence, without pay, for the period of the employee's assignment as a representative of such employee organization. Such employee shall, however, be credited with the creditable service the employee would have received had the employee been in active service for the full or part-time leave and shall contribute each month to the retirement fund in an amount which the employee would have contributed had the employee remained in the service of the commonwealth or its political subdivisions. Such employee of the commonwealth or its political subdivisions shall be entitled to all benefits and privileges, except the payment of salary as provided under this chapter and chapters 30 and 31 during the leave of absence.
An employee who fulfills the requirements of the preceding paragraph shall be credited with creditable service for any period after January 1, 1975 and shall contribute to the retirement fund an amount which he would have contributed had such employee remained in the service of the commonwealth or a political subdivisions thereof, together with regular interest thereon, under the terms and conditions defined by the retirement system of which he is a member. This paragraph shall take effect for the members of a retirement system by majority vote of the board of such system, subject to the approval of the legislative body. For the purposes of this section, "legislative body" shall mean the town meeting for the purposes of a town system, the city council subject to the provisions of its charter in a city system, the district meeting in a district system, the county commissioners in a county system and the governing body of an authority in an authority system. Acceptance shall be deemed to have occurred upon the filing of a certificate of acceptance with the commission.
Mass. Gen. Laws ch. 32, § 28K