Within any planning region of the state a regional council of governments may be created by the adoption of sections 4-124i to 4-124p, inclusive, by ordinance of the legislative bodies of not less than sixty per cent of all towns, cities and boroughs within such planning region entitled to membership on such council as hereinafter provided. All towns, cities and boroughs within a planning region shall be entitled to membership on such council, including any city or borough with boundaries not coterminous with the boundaries of the town in which it is located. Any nonmember town, city or borough entitled to membership may join the council by the adoption of said sections by ordinance of its legislative body. Any member town, city or borough may withdraw from the council by adoption of an appropriate ordinance of its legislative body to become effective on the date of such adoption; provided, however, that any such withdrawing member shall be obligated to pay its pro rata share of expenses of operation and pro rata share of funds committed by the council to active programs as of such date of withdrawal.