Mass. Gen. Laws ch. 40B § 3
Any group of cities, towns, or cities and towns may, by vote of their respective city councils or town meetings, vote to become members of and thus establish a planning district, which shall constitute a public body corporate. After a planning district has been thus established, any other city or town within the district area as hereinafter defined may by vote of its city council or town meeting apply for admission. Upon the affirmative vote of two thirds of the representatives of the cities and towns comprising the district, said city or town shall become a member thereof. The area of jurisdiction of said district shall be an area defined or redefined as an effective regional planning region by the Massachusetts office of business development. All rights, privileges and obligations applicable to the original members of the district shall be applicable to the new members.
Mass. Gen. Laws ch. 40B, § 3