noun
senior member; old-timer; veteran
Refers to a person who has been in a group, organization, or place for a long time, often with informal seniority.
彼はこの会社の古参者で、もう二十年も勤めている。
He is an old-timer at this company, having worked here for twenty years.
古参者の意見はやはり重みがある。
The opinions of senior members naturally carry weight.
先輩 is a more common and broader term for a senior in school, work, or any group, while 古参者 emphasizes long tenure and experience.
Compound of 古参 (こさん, 'long service, seniority') and 者 (しゃ, 'person').