From the FAQs

Graduate Faculty membership is only intended for faculty members who will participate broadly in the training of graduate students on an ongoing basis.

All members of the Graduate Faculty must be nominated by the graduate program to which they will contribute, and must be current, full-time faculty members of the University holding an academic rank as defined in AC-21.

Faculty members who are intended to only teach graduate courses, or serve on the occasional doctoral committee should not be nominated to the Graduate Faculty; such individuals should seek permission to teach specific graduate courses, or be nominated to serve as Special Member of a doctoral committee.

Individuals holding adjunct, affiliate, visiting, and part-time appointments should not be nominated for membership in the Graduate Faculty. Finally, Graduate Faculty membership is not a way to recognize individuals who hold graduate degrees but do not participate broadly and on a continuing basis in graduate education.