A person involved in or connected to an event, matter, or organization; those concerned or authorized (often used in notices like 'authorized personnel only').
事故現場には関係者以外立ち入り禁止です。
Only authorized personnel may enter the accident site.
関係者にのみ詳細を連絡した。
I informed only the people involved of the details.
会議は関係者だけで行われた。
The meeting was held only with those concerned.