joiner vs fellow member

joiner and fellow member both are nouns.

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

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

joiner: a person who likes to join groups
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 joiner include member.

joiner (noun)	fellow member (noun)
a woodworker whose work involves making things by joining pieces of wood	one of the persons who compose a social group (especially individuals who have joined and participate in a group organization)
a person who likes to join groups