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 Actors Branch of the Academy, a candidate must:
(a) have a minimum of three theatrical feature film credits, in all of which
the roles played were scripted roles, one of which was released in the past
five years, and all of which are of a caliber that reflect the high standards
of the Academy,
(b) have been nominated for an Academy Award in one of the acting categories,
(c) have, in the judgment of the Actors Branch Executive Committee,
otherwise achieved unique distinction, earned special merit or made an outstanding contribution as a motion picture actor.