10 U.S.C. § 173: Advisory personnel

(a) The Secretary of Defense may establish such advisory committees and employ such part-time advisers as he considers necessary for the performance of his functions and those of the agencies under his control. (b) A person who serves as a member of a committee may not be paid for that service while holding another position or office under the United States for which he receives compensation. Other members and part-time advisers shall (except as otherwise specifically authorized by law) serve without compensation for such service.