Cal. Pub. Util. Code § 99563

Transit district employees shall have the right to form, join, and participate in the activities of employee organizations of their own choosing for the purpose of representation on all matters of employer-employee relations and for the purpose of meeting and conferring and shall have the right to engage in other concerted activities for the purpose of collective bargaining or other mutual aid or protection. Transit district employees shall also have the right to refuse to join employee organizations or to participate in the activities of these organizations subject to the organizational security provision permissible under this chapter.