California Code, Government Code - GOV § 20502

(a)(1) The initial contract shall include in this system all firefighters, police officers, county peace officers, local sheriffs, and other employees of the contracting agency, except as exclusions in addition to the exclusions applicable to state employees may be agreed to by the agency and the board. The contract shall not provide for the exclusion of some, but not all, firefighters, police officers, county peace officers, or local sheriffs. The exclusions of employees, other than firefighters, police officers, county peace officers, or local sheriffs, shall be based on groups of employees such as departments or duties, and not on individual employees.

(2) The exclusions of groups shall not be made by amendments to contracts.An amendment to a contract to enumerate or clarify provisions related to groups of employees in a manner that does not expand those already subject to exclusion shall not be considered an exclusion prohibited by this paragraph.

(3) Membership in this system is compulsory for all employees included under a contract.