Sentence:  Not only should every educator above the rank of instructor be expected to be a <WSD>member</WSD> of one of the professional organizations , but his first qualification for membership as an educator should be so sharply scrutinized that membership would be equivalent to certification to teach the subject . 
Meaning of the word marked with <WSD> tags:  one of the persons who compose a social group (especially individuals who have joined and participate in a group organization)