Party joinder power allows combining or substituting parties when common questions arise and participation is necessary for determination. Regulation 25 empowers the Commission to join multiple persons or enterprises as parties or to substitute parties where rights arise from the same act or transaction, a common question of law or fact exists, and participation is necessary for determination; substitution may be ordered when necessary, admitted parties may obtain certified document copies from the Secretary within ten days, and such orders do not affect prior procedural steps.