们 = 亻(person) + 门 (door, giving the sound 'men'). The person radical signals it attaches to people. It's a grammatical suffix, not a word you use alone.
们 is the closest Chinese gets to a plural. Add it to a pronoun and you pluralize: 我 (I) → 我们 (we), 你 (you) → 你们 (you all), 他 (he) → 他们 (they). It's toneless (men, light and quick). Chinese nouns otherwise don't change for plural — 书 means 'book' or 'books', context decides — so 们 is special, reserved mostly for people.
A person (亻) at the door (门) — and everyone else too: the PLURAL marker.
们 only pluralizes people, not things. 三本书 is 'three books' (no 们). But 老师们 = 'teachers', 朋友们 = 'friends'. Pronounced light and toneless: men.