The applicant may be represented before the Commission by any person who is a member in good standing of the bar of the Supreme Court of the United States or of the highest court of any State or territory of the United States, or the District of Columbia, and who is not under any order of any court suspending, enjoining, restraining, or disbarring him from, or otherwise restricting him in, the practice of law. Whenever a person acting in a representative capacity appears in person or signs a paper in practice before the Commission, his personal appearance or signature shall constitute a representation to the Commission that under the provisions of this part and applicable law he is authorized and qualified to represent the particular person in whose behalf he acts. Further proof of a person's authority to act in a representative capacity may be required. When any applicant is represented by an attorney at law, any notice or other written communication required or permitted to be given to or by such applicant shall be given to or by such attorney. If an applicant is represented by more than one attorney, service by or upon any one of such attorneys shall be sufficient.