40 ILCS 5/15-134

(a) Each person shall, as a condition of employment, become a participant and be subject to this Article on the date that he or she becomes an employee, makes an election to participate in, or otherwise becomes a participant in one of the retirement programs offered under this Article, whichever date is later. An employee who becomes a participant shall continue to be a participant until he or she becomes an annuitant, dies or accepts a refund of contributions. (b) A person employed concurrently by 2 or more employers is eligible to participate in the system on compensation received from all employers.