Tenn. Code Ann. § 8-35-112: Membership in retirement system as condition of employment

(a) Notwithstanding any other law to the contrary and except as provided in subsection (b), any current or former member of the retirement system or of a superseded system who accepts, or is elected to, a position on or after July 1, 2018, for which membership in the retirement system is otherwise optional shall become a member of the retirement system as a condition of employment.

(b) Subsection (a) shall not apply to an employee having optional membership who was employed by a political subdivision on the date the political subdivision elected to extend retirement coverage to the employee, unless the employee was a member or former member of a preexisting defined benefit plan maintained by that political subdivision. For purposes of this section, "political subdivision" means any entity authorized to participate in the retirement system pursuant to part 2 of this chapter.

(c) This section does not apply to retired members of the retirement system or of a superseded system who return to service in a position covered by the retirement system, as provided in chapter 36, part 8 of this title that does not allow the retired member to accrue additional retirement benefits.