Refers to a person who provides support, often financial or organizational, to a cause, project, or organization. Commonly used in formal or institutional contexts.
He is active as a supporter of that charity organization.
The project was realized thanks to donations from backers.
後援者 also means supporter or patron, but 賛助員 is more commonly used for members of a supporting organization or committee.
Compound of 賛助 (sanjō, 'support, backing') and 員 (in, 'member, person'). The exact historical derivation is uncertain, but the word follows a common pattern for forming nouns denoting a person with a specific role.