Cal. Gov. Code § 31800

The provisions of this article shall be applicable to any member who is subject to the federal old age and survivors insurance provisions of the Social Security Act, when the governing board of the county or district in which the member is employed adopts by majority vote a resolution providing that this article shall be applicable to all members in such county or district who are subject to the federal system. The provisions of this article shall become fully effective and operative on the date specified in such resolution; provided, however, such resolution shall have received prior approval by majority affirmative vote of eligible members employed by the county or district in a referendum conducted in accordance with the provisions of Article 2, Chapter 2, Part 4, Division 5, of Title 2 of this code. Nothing in this article shall be construed as negating or in any way affecting the validity of a referendum vote conducted prior to the enactment of this article, whereby a majority of members employed by a county or district voted in favor of federal old age and survivors insurance coverage on a purely additive or supplemental basis.