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.
Members of the Short Films and Feature Animation Branch are filmmakers, artistic craftspeople,
and those who have made notable contributions to the field. To be considered for invitation to membership, an individual must:
(a) have at least two theatrical film screen credits in a key creative role* of a caliber which, in the opinion of the Short Films and Feature Animation Branch Executive Committee, reflect the high standards of the Academy,
(b) have been nominated for an Academy Award in a Short Films category
(theatrical live action and animated short films), or the Animated Feature Film category.
(c) have, in the judgment of the Short Films and Feature Animation Branch Executive Committee, otherwise achieved unique distinction, earned special merit or made an outstanding contribution within the person's field of endeavor.
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.
* Candidates may be considered if they are key to the creative core of a film in the following categories:
Live Action Short Film: Producer, Director, Writer.
Animated Short and Feature Film: Producer, Director, Artist or Technical Artist in a Creative
Supervising, Directing or Lead Role.