(a) The cooperating cities and counties which join to create a metropolitan or regional planning commission, through joint agreement, shall determine the number and qualifications of the members of the commission.
(b) The joint agreement shall also provide for the manner of cooperation and the means and methods of the operation and functioning of the commission, including the employment of a director of planning and such staff and consultants as it may require, the proportionate share of costs and expenses, and the purchase of property and materials for the use of the commission.
(c) The joint agreement may also allow for the addition of other public bodies to the cooperative arrangement.