Cal. Gov. Code § 21099

This article shall not apply to a contracting agency nor its employees until, first, it is agreed to in a written memorandum of understanding entered into by an employer and representatives of employees and, second, the contracting agency elects to be subject to it by amendment to its contract made in the manner prescribed for approval of contracts or in the case of a new contract, by express provision of the contract. The operative date of this article with respect to a local miscellaneous member shall be the effective date of the amendment to his or her employer’s contract electing to be subject to this article.