participant; party concerned
Formal term for a person or entity involved in an event, project, or legal matter.
All participants in this project are bound by confidentiality.
He was questioned as a party concerned in the accident.
関係者 is broader, meaning 'person concerned' or 'stakeholder', while 関与者 specifically refers to someone actively involved or participating.
Compound of 関与 (involvement, participation) + 者 (person). The exact historical derivation is uncertain, but the word follows a common Sino-Japanese pattern for forming agent nouns.