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 Casting Directors Branch of the Academy, an individual:
(a) must have a minimum of eight years experience as Casting Director on theatrical feature films,
(b) be primarily responsible for and have screen credit as Casting Director on at least ten theatrical feature films of a caliber which, in the opinion of the Review Committee, reflect the high standards of the Academy,
(c) have, in the judgment of the Review Committee, otherwise achieved unique distinction, earned special merit or made an outstanding contribution as a Casting Director.
Letters of recommendation are neither required, nor will they be accepted.