During the live steaming event, I took the words from Kim Roberts Hedgepath (AFTRA’s National Executive Director) who indicated that the union’s constitution mandates an open union.
For clarification, here is the text from the proposed new union constitution as it refers to membership:

Interpret as you will:

Article III Membership
     A.     Qualifications for Membership
                         1.A person shall be eligible for membership in SAG-AFTRA if her or she:
                            a. Has worked or is working or is about to work in a position covered by a SAG-AFTRA or AFTRA or SAG collective bargaining agreement., provided that any person qualifying for work as a background actor must have completed three(3) days of work as a background actor under a  SAG-AFTRA or AFTRA or SAG collective bargaining agreement; or
                              b. Is determined by the National Board to be engaged in work that advances the active organizing efforts or general goals of SAG-AFTRA.