commissioner vs fellow member

commissioner and fellow member both are nouns.

Word	Noun	Adjective	Verb	Adverb
commissioner	Yes	No	No	No
fellow member	Yes	No	No	No

As nouns, fellow member is a hypernym of commissioner; that is, fellow member is a word with a broader meaning than commissioner:

commissioner: a member of a commission
fellow member: one of the persons who compose a social group (especially individuals who have joined and participate in a group organization)

Other hypernyms of commissioner include member.

commissioner (noun)	fellow member (noun)
a government administrator	one of the persons who compose a social group (especially individuals who have joined and participate in a group organization)
a member of a commission