Mass. R. Dom. Rel. P. Rule 24
(a) Intervention of Right. Upon timely application anyone shall be permitted to intervene in an action: (1) when a statute of the Commonwealth confers an unconditional right to intervene or (2) when the applicant claims an interest relating to the property or transaction which is the subject of the action and he is so situated that the disposition of the action may as a practical matter impair or impede his ability to protect that interest, unless the applicant's interest is adequately represented by existing parties. Identical to Mass.R.Civ.P. 24(a).
(b) Permissive Intervention. Upon timely application anyone may be permitted to intervene in an action: (1) when a statute of the Commonwealth confers a conditional right to intervene; or (2) when an applicant's claim or defense and the main action have a question of law or fact in common. When a party to an action relies for ground of claim or defense upon any statute or executive order administered by a federal or state governmental officer or agency or upon any regulation, order, requirement, or agreement issued or made pursuant to the statute or executive order, the officer or agency upon timely application may be permitted to intervene in the action. In exercising its discretion the court shall consider whether the intervention will unduly delay or prejudice the adjudication of the rights of the original parties. Identical to Mass.R.Civ.P. 24(b).
(c) Procedure. A person desiring to intervene shall serve a motion to intervene upon the parties as provided in Rule 5. The motion shall state the grounds therefor and shall be accompanied by a pleading setting forth the claim or defense for which intervention is sought. Identical to Mass.R.Civ.P. 24(c).
(d) Intervention by the Attorney General. When the constitutionality of an act of the legislature or the constitutionality or validity of an ordinance of any city or the by-law of any town is drawn in question in any action to which the Commonwealth or an officer, agency, or employee thereof is not a party, the party asserting the unconstitutionality of the act or the unconstitutionality or invalidity of the ordinance or by-law shall notify the attorney general within sufficient time to afford him an opportunity to intervene. Identical to Mass.R.Civ.P. 24(d).