California Code, Government Code - GOV § 53220

Unless the legislative body of the local agency specifically provides that a pension trust plan established pursuant to this article is compulsory upon the officers and employees of the local agency, it shall not be compulsory for an officer or employee of such agency to accept or join in such plan.

If, pursuant to Section 53216, the legislative body of the local agency makes such plan compulsory, it may provide that every officer and employee of the local agency who is eligible for membership, except for elective officers, shall on the start of the first pay period following the effective date of this section automatically become a member of said plan; and, that thereafter each other employee and officer of said agency who is eligible for membership in said plan under its terms, except for elective officers, automatically becomes a member of said plan on the day of his entry into employment with said agency. The legislative body of the local agency may provide that elective officers of said agency become members of said plan either on the first day of taking office or on the first day of the payroll period following the filing of a declaration by such an officer with the board of trustees of the pension trust to become a member.