Ind. Code § 8-1-3-3
Any person firm, association, corporation, limited liability company, city, town or public utility may file with the clerk of the court a verified petition to be made a party appellant or appellee, which petition shall allege facts showing that the petitioner has a substantial interest in the determination of the action, and such petitioner shall be made a party appellant or appellee as its interest appears. Any party applicant, intervenor or protestant in the proceedings had before the commission in the matter from which the appeal is taken shall be and have the rights of a party on appeal, upon the filing of a written appearance therein. But no party by his appearance or petition to be made a party shall acquire the right to file a brief, the time for filing of which has expired prior to his appearance.
IC 8-1-3-3