20 C.F.R. § 725.363

(a) Attorney. Any attorney in good standing who is admitted to practice before a court of a State, territory, district, or insular possession, or before the Supreme Court of the United States or other Federal court and is not, pursuant to any provision of law, prohibited from acting as a representative, may be appointed as a representative.

(b) Other person. With the approval of the adjudication officer, any other person may be appointed as a representative so long as that person is not, pursuant to any provision of law, prohibited from acting as a representative.