Joinder and substitution of parties permitted where common questions arise and participation is necessary for issue determination. The Commission may join multiple persons or enterprises as parties where they claim relief arising from the same act or transaction, a common question of law or fact exists, and the participation of those persons or enterprises is necessary for determination of the issues. The Commission may substitute parties when required. Persons admitted by joinder or substitution may, on application, receive copies of prior filings as determined by the Commission, and orders for joinder or substitution do not affect prior steps in the proceedings.