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 Academy membership in the Documentary branch of the Academy, a candidate must:
(a) have a minimum of two director and/or producer credits on theatrical documentary films (one of which must have been within the last five years) of a caliber which, in the opinion of the executive committee, reflect the high standards of the Academy,
(b) have director and/or producer screen credit on a picture nominated for an Academy Award in the Documentary category,
(c) have, in the judgment of the Documentary Executive Committee, otherwise achieved unique distinction, earned special merit and made an outstanding contribution to theatrical documentary filmmaking.
Proposals must be accompanied by a letter from each sponsor which addresses, as specifically as possible, how the candidate meets one or another of the requirements above.