noun
supporter; person in favor
Refers to someone who agrees with or supports a proposal, plan, or opinion. Often used in formal or voting contexts.
その提案には賛成者が多かった。
There were many supporters of that proposal.
賛成者は手を挙げてください。
Those in favor, please raise your hands.
The noun/verb 'agreement' or 'approval'; 賛成者 is the person who gives that approval.
Compound of 賛成 (さんせい, 'agreement, approval') and 者 (しゃ, 'person').