People are (naturally) influenced by the people around them
「人は周りの人の影響を受けるものだ 」は英語で People are (naturally) influenced by those around them もしくは People are (naturally) influenced by the people around them になると思います。
この場合は People はいいと思いますが、一般論を言う時は humans を使う場合もあります。
As humans we are naturally influenced by the people around us / As humans we are naturally influenced by the people we associate ourselves with もいいと思います。
影響する - to influence
naturally - 自然に
associate - 付き合う