From Academy Bylaws:
Article III, Section 1. Membership shall be by invitation of the Board of Governors. Invitations to active membership shall be limited to those persons active in the motion picture arts and sciences, or credited with screen achievements, or who have otherwise achieved distinction in the motion picture arts and sciences and who, in the opinion of the Board, are qualified for membership.
To be considered for invitation to membership in the Directors Branch of the Academy, a candidate must:
(a) have a minimum of two directorial credits, at least one of which is from the most recent ten years, on theatrical feature films of a caliber which, in the opinion of the executive committee, reflect the high standards of the Academy,
and/or
(b) have directorial screen credit on a picture nominated for the Academy Directing Award, for the Academy Best Picture and/or for the Academy International Feature Film Award.
or
(c) have, in the judgment of the Directors Branch Executive Committee, otherwise achieved unique distinction, earned special merit or made an outstanding contribution as a motion picture director.