Mo. Rev. Stat. § 86.1480: Who shall be members.

1. Every person who becomes an employee, as defined in subdivision (9) of section 86.1310, after August 28, 2001, shall become a member of the retirement system defined in sections 86.1310 to 86.1640 as a condition of such employment.

2. Every person who was a member of the retirement system on or before August 28, 2001, shall remain a member.

3. Every person who was an employee, as defined in subdivision (9) of section 86.1310, on August 28, 2001, but was not a member, shall become a member as a condition of employment upon the completion of six months' continuous employment.