reserve member; alternate member
Refers to a person appointed as a backup or alternate member of a committee, board, or similar body, ready to serve when a regular member is unavailable.
He was selected as a reserve member of the committee.
委員 is a regular committee member, while 予備委員 is a reserve or alternate member who steps in when needed.
Compound of 予備 (yobi, 'reserve, spare') and 委員 (iin, 'committee member').