Additional municipalities within or outside the state may join the system, as provided in N.H. Rev. Stat. § 154:30-a through N.H. Rev. Stat. § 154:30-h, and shall be received as members subject to the approval of the board of directors. Municipalities which do not have active fire departments may be admitted as members upon such conditions as the board of directors may fix. Private fire departments within or outside the state may also be accepted as members, with equal voting rights, by the board of directors, under such arrangements as are mutually agreed upon. A municipality or private fire department may, by vote of its governing board, withdraw from the system but such withdrawal shall not be effective until 90 days after written notice of such withdrawal shall have been delivered to one of the officers of the system.