expression, noun
member of one's in-group; one of us
Refers to someone belonging to the speaker's family, company, or other close social group. Often used when distinguishing insiders from outsiders.
内の者に確認してから折り返しご連絡いたします。
I will check with someone in our office and get back to you.
この件は内の者だけが知っ
Only people inside our group know about this matter.
身内 is a more general term for one's close relatives or inner circle, while 内の者 explicitly refers to a specific person belonging to that group.
仲間 means companion or colleague and can include peers, whereas 内の者 emphasizes the insider/outsider boundary.
Compound of 内 (uchi, 'inside, one's own group') + の (no, possessive particle) + 者 (mono, 'person'). The phrase literally means 'person of the inside'.