(a) A combination of local agencies may form a groundwater sustainability agency by using any of the following methods:
(1) A joint powers agreement.
(2) A memorandum of agreement or other legal agreement.
(b) A water corporation regulated by the Public Utilities Commission or a mutual water company may participate in a groundwater sustainability agency through a memorandum of agreement or other legal agreement. The authority provided by this subdivision does not confer any additional powers to a nongovernmental entity.