10 U.S.C. § 12107

(a) Except as provided in subsection (c), to become an enlisted member of the Army National Guard of the United States or the Air National Guard of the United States, a person must— (1) be enlisted in the Army National Guard or the Air National Guard, as the case may be; (2) subscribe to the oath set forth in section 304 of title 32 ; and (3) be a member of a federally recognized unit or organization of the Army National Guard or the Air National Guard, as the case may be, in the grade in which he is to be enlisted as a Reserve. (b) (1) Under regulations to be prescribed by the Secretary of the Army, a person who enlists in the Army National Guard, or whose term of enlistment in the Army National Guard is extended, shall be concurrently enlisted, or his term of enlistment shall be concurrently extended, as the case may be, as a Reserve of the Army for service in the Army National Guard of the United States. (2) Under regulations to be prescribed by the Secretary of the Air Force, a person who enlists in the Air National Guard, or whose term of enlistment in the Air National Guard is extended, shall be concurrently enlisted, or his term of enlistment shall be concurrently extended, as the case may be, as a Reserve of the Air Force for service in the Air National Guard of the United States. (c) (1) A member of the Army Reserve who enlists in the Army National Guard in his reserve grade, and is a member of a federally recognized unit or organization of the Army National Guard, becomes a member of the Army National Guard of the United States and ceases to be a member of the Army Reserve. (2) A member of the Air Force Reserve who enlists in the Air National Guard in his reserve grade, and is a member of a federally recognized unit or organization of the Air National Guard, becomes a member of the Air National Guard of the United States and ceases to be a member of the Air Force Reserve.