In what circumstances will the trial judge allow the parents of a plaintiff to participate in the trial?

In McGuckin v. McGuckin, supra, a wife brought an action for division of matrimonial property. The home in which she and her husband had lived was registered in the names of the husband's parents. The wife alleged that the home was matrimonial property. No application was brought to add the parents as third parties, but the trial judge permitted them to participate fully in the trial (which was tantamount to giving them the status of parties) and in doing so said this at p. 112: "... It is obviously necessary to determine what is matrimonial property, and to do so, I must hear the registered owners who say that it is not matrimonial property of the applicant but their property. The registered owners took part in the proceedings throughout; they had counsel, called witnesses and cross-examined the applicant's witnesses, and in every respect acted as parties. ..."