Cal. Gov. Code § 3599.57

Employee organizations have the right to represent their members in their employment relations with the employer, except that once an employee organization is recognized as the exclusive representative of an appropriate unit, the recognized employee organization is the only organization that may represent that unit in employment relations with the employer. Employee organizations may establish reasonable restrictions regarding who may join and may make reasonable provisions for the dismissal of individuals from membership. This section does not prohibit any employee from appearing on the employee’s own behalf in the employee’s employment relations with the employer.