(a) Except as otherwise provided by this subchapter, a person who is not a member becomes a member of the retirement system if:
(1) on the date a municipal department's participation in the retirement system becomes effective, the person is an employee of the department;
(2) after August 31, 1987, the person becomes an employee of a participating department; or
(3) the person on August 31, 1987, is an employee of a participating department but is not a member because at the time of employment the person's age exceeded the maximum age for becoming a member.
(b) Any person to whom Subsection (a)(1) applies becomes a member of the retirement system on the date the department's participation becomes effective, and any person to whom Subsection (a)(2) applies becomes a member of the retirement system on the date the person is employed. A person to whom Subsection (a)(3) applies becomes a member of the retirement system on September 1, 1987.