"(1) The Upper Tribunal may give a direction adding, substituting or removing a party as an appellant, a respondent or an interested party. (2) If the Upper Tribunal gives a direction under (1) it may give such consequential directions as it considers appropriate. (3) A person who is not a party may apply to the Upper Tribunal to be added or substituted as a party."
"(a) the person who starts proceedings (whether by bringing or notifying an appeal, by making an originating application, by reference, or otherwise), and (b) in proceedings started jointly by more than one person such persons acting jointly or each such person as the context requires; (c) a person substituted as an appellant under rule 9 ..."
"whose presence before the court is necessary to ensure that all matters in dispute in the cause or matter may be effectively and completely determined and adjudicated upon."