Tenn. Code § 8-3602
Membership in the retirement system for employees who are admitted as provided in § 8-3601 shall be optional with such employees in the service of the political subdivision on the date the approval is given, and any such employee who elects to join the retirement system within thirty (30) days thereafter shall be entitled to a prior service certificate covering such periods of previous service as shall be certified as creditable service by such political subdivision for service rendered to such political subdivision, or its predecessor, or in any other capacity approved by the political subdivision and the board, for which the political subdivision is willing to make accrued liability contributions. Thereafter service for such political subdivision on account of which the political subdivision pays contributions, shall be considered also as creditable service. Membership shall be compulsory for all employees entering the service of such political subdivision thereafter. Membership shall be compulsory for employees of the University of Tennessee entering service after the date approval is given by the board, except those employees who are to be included in the University of Tennessee retirement system.
T.C.A. § 8-3602