California Code, Water Code - WAT § 81456

(a) A public entity identified in Section 81305 that is not a member at the time the agency is formed pursuant to Section 81328 may thereafter join the agency, with the approval of the board of the agency and subject to those terms and conditions that the board determines are necessary to ensure that the new member pays an equitable share of the costs previously incurred by the agency.

(b) The new member shall appoint one director and, for the purposes of this division, the new member shall be subject to the water delivery amounts described in Section 81460 that apply to that new member.