Any political subdivision may become a member of any existing authority, and any political subdivision which is a member of an existing authority may withdraw therefrom, but no political subdivision shall be permitted to withdraw from any authority after any obligation has been incurred by the authority except by unanimous vote of all members of the authority. The governing body of any political subdivision wishing to withdraw from an existing authority shall signify its desire by resolution or ordinance. The governing body of any political subdivision wishing to become a member of an existing authority and the governing bodies of the political subdivisions then members of the authority shall by concurrent resolutions or ordinances or by agreement provide for the joinder of such political subdivision and specify the number and term of office of members of the expanded authority which are to be appointed by each of the participating political subdivisions, together with the name, address and term of office of initial appointments to membership.