What are the nuances between 一員, 会員, 成員, and メンバー? There is a goo thesaurus page, but it didn't really help me actually distinguish the differences.
Somewhat strangely, メンバー may be the most neutral and versatile. Also it sometimes means regular members in sports like メンバーから外される (removed from regular lineup).
会員's basic connotation is being registered or listed in a group of people. For example, a user of a credit card is called カード会員; using amazon prime means you are a アマゾンプライム会員 (though non-prime users won't be usually called アマゾン会員). Of course, it is used to mean a member of groups with name ・・・の会.
成員 is also a member, where the group of people is less specific (less clearly defined). It means 'a member constituting a body' rather than 'registered member', and sounds more formal than the others. For example, 社会の成員 or 組織の成員 means a member of society/organization. Here 会員 is not possible and using メンバー is less idiomatic.
一員 is exactly like a member with indefinite article. Basically it is the same usage as 一個人, emphasizing 'one-of-the many' nature of the person. It means the same as 成員, but 一員 is usually not a subject.
- 社会の一員/成員になる become a member (part) of the society
- 組織の各成員が全力を尽くす Each member of the organization does her best. 各一員 is not possible.
It is often used with として: 会社の一員としての責任 responsibility as a member of the company.