a proverb meaning 'birds of a feather flock together' — people with similar traits tend to gather together.
類は友を呼ぶと言うけれど、彼らのグループを見ると確かにそうだ。
They say 'birds of a feather flock together,' and looking at their group you can see it's true.
趣味が似ている人が集まるのは、まさに類は友を呼ぶだ。
People with similar hobbies gather — it's exactly a case of 'birds of a feather flock together.'