53 Pa. Stat. § 881.107
Any municipality may elect, by ordinance or resolution adopted by the tax levying body, or in the case of municipal authorities by the board of such municipal authority to join the system: Provided, however, That any municipality electing coverage under the provisions of Article II of this act must have first placed its municipal employes in so far as they are eligible under the Federal Social Security Act.
Any municipality, by action of its tax levying body, may and upon petition of electors equal to at least five per cent of the registered electors of the municipality, shall, submit the question of joining such system to the voters of the municipality at any municipal or general election, in the same manner as other questions are submitted to the electors under the election code of the Commonwealth. If the majority of the electors voting on the question vote in favor thereof, the tax levying body shall adopt an ordinance or resolution electing to join such system. If the electors vote against joining the system, then no further action shall be taken in the municipality for a period of two years.
A duly certified copy of any such ordinance or resolution electing to join the system shall be filed with the board.
53 P.S. § 881.107