(a) A petition for administrative review (PAR) of an order issued by a rent administrator may be filed by a party to the proceeding, or other necessary party, in the manner provided in this Part, where such petition alleges the errors upon which such order is based.

1. (1) A joint PAR, verified or affirmed by each person joining therein, may be filed by two or more owners or tenants, where at least one ground is common to all persons so filing. The commissioner, in his discretion, may treat such PAR as joint or several.
2. (2) A PAR may also be filed by a representative of a party, including an attorney at law, provided that said representative duly verifies or affirms the PAR and provided that such representative includes, at the time of the filing of the PAR, written evidence of authorization to act in such representative capacity for the purpose of filing the PAR.

(b)