Is it permissible for a person and his parents to share in sponsoring a single orphan in order to attain the reward of accompanying the Prophet, may Allah’s prayers and peace be upon him, or is the reward exclusive to the sponsor only?

It is permissible to sponsor an orphan and to include one's parents or others in the reward, for what is designated for them of the reward will benefit them. As for companionship with the Prophet, peace and blessings be upon him, in Paradise, that is for whoever actually undertakes the sponsorship of the orphan, unless they share in the costs of sponsorship. The aforementioned hadith (definition) is authentic: "I and the sponsor of the orphan will be in Paradise like this," and in a narration by Muslim: "The sponsor of the orphan, whether the orphan is his relative or not, I and he are like these two in Paradise."