Practice before the board in informal proceedings shall be limited to the following:

(1) Taxpayers who are natural persons representing themselves;

(2) Attorneys at law duly qualified and entitled to practice in the courts of the state of Washington;

(3) Public officials in their official capacity;

(4) Certified public accountants licensed in the state of Washington;

(5) A duly authorized director, officer, or full-time employee of an individual firm, association, partnership, or corporation who appears for such firm, association, partnership, or corporation;

(6) Partners, joint venturers, or trustees representing their respective partnerships, joint venturers, or trusts; and

(7) Other persons designated by a taxpayer with approval of the board.