All persons appointed fire fighters to the Woonsocket fire department after July 1, 1985, are a group under this retirement system. Upon the effective date of participation of the city of Woonsocket in this retirement system any fire fighter appointed after July 1, 1985, waives and renounces all accrued rights and benefits of any other pension or retirement system supported wholly or in part by a participating municipality, becomes a member of this retirement system and is not required to make a contribution under any other pension or retirement system of a participating municipality, anything to the contrary notwithstanding; provided, that for each fire fighter appointed after July 1, 1985 and prior to July 1, 1989, the city of Woonsocket contributes both the employee‘s and the employer’s share of the cost of the service credit, plus interest as determined by the board. The city of Woonsocket may from time to time by notice to the board add any fire fighter appointed to the Woonsocket fire department on or before July 1, 1985 to the group, and the fire fighter becomes a member of the group in the same manner as provided in this section for fire fighters appointed after July 1, 1985 and prior to July 1, 1989.