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 Costume Designers Branch of the Academy, an individual must:
(a) have screen credits as Costume Designer on at least six feature films of a caliber which, in the opinion of the executive committee, reflect the high standards of the Academy,
(b) have screen credit on a picture nominated for the Costume Design Award,
(c) have, in the judgment of the Costume Designers Branch Executive Committee, otherwise achieved unique distinction, earned special merit or made an outstanding contribution within his or her field of endeavors.
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.