Cal. Gov. Code § 3531

Supervisory employees shall have the right to form, join, and participate in the activities of supervisory employee organizations of their own choosing for the purpose of representation on all matters of supervisory employer-employee relations, as set forth in Section 3532. Supervisory employees also shall have the right to refuse to join or participate in the activities of supervisory employee organizations and shall have the right to represent themselves individually in their employment relations with the public employer.