Refers to a member of a council or board, typically in an organizational or governmental context. The term is relatively formal and not an everyday word.
He was elected as a city councilor.
議員 is a more common and general term for a member of an assembly or parliament, while 参事会員 is specifically a member of a council or board, often with an advisory or administrative role.
Compound of 参事 (さんじ, 'councilor' or 'advisor') + 会員 (かいいん, 'member'). The exact historical derivation is uncertain, but the term follows standard Sino-Japanese word formation.