们【men】

们 makes a pronoun or a word for people plural. It attaches to the end and is toneless.

Only for people

们 does not go on things. Chinese nouns have no plural form at all – 书 is "book" or "books", and the number comes from the rest of the sentence. Sān三běn本Shū书men们 is wrong.

A number blocks 们

Even with people, 们 cannot appear once a quantity is stated. Sān三gě个xué sheng学生men们 is wrong, because the number already marks plurality.

It is optional and often left out

With a plain noun, 们 adds a sense of the group taken together, and it is common in address – 老师们, 朋友们, 先生们. Outside address it is left out more often than not, and 学生 alone covers both numbers.