Md. Code, State Personnel and Pensions § 28–202

(a) Except as provided in subsection (b) of this section, an individual described in § 28-201 of this subtitle is a member of the Local Fire and Police System as a condition of employment.

(b) Membership in the Local Fire and Police System is optional for an individual who was employed by a participating governmental unit on the date the participating governmental unit begins to participate in the Local Fire and Police System.