sodalist vs fellow member

sodalist and fellow member both are nouns.

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

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

sodalist: a member of a sodality
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 sodalist include member.

sodalist (noun)	fellow member (noun)
a member of a sodality	one of the persons who compose a social group (especially individuals who have joined and participate in a group organization)