noun
supporter; advocate; proponent
A person who agrees with and supports a cause, idea, or proposal. Often used in formal or organizational contexts.
彼はその計画の賛同者を集めている。
He is gathering supporters for the plan.
賛同者の署名が必要です。
Signatures of supporters are required.
賛成者 also means 'supporter' but is more commonly used for voting or formal approval; 賛同者 implies a deeper alignment of opinion.
Compound of 賛同 (agreement, approval) + 者 (person). The exact historical derivation is straightforward.