an adjunct to a pronoun or noun to indicate plurality
你 (you) is a singular pronoun in Chinese, Adding 們 after it make it a plural pronoun- 你們(you)
我 (I / me) is a singular pronoun in Chinese. Adding 們 after it, makes it a plural pronoun- 我們 (we / us)
他 (he / him) is a singular pronoun in Chinese. Adding 們 after it, makes it a plural pronoun- 他們 (they / them)
The adjunct 們 is mostly used with human pronouns (e.g. - 你們, 我們, 他們). Although it can be used with a noun, it is restricted to nouns for human only.
戰士 warrior / 戰士(們) warrior(s)
學生 student / 學生(們) student(s)
工人 worker / 工人(們) worker(s)
人 person ; people / 人們 (people) * 人 can be either singular or plural
Using 們 with nouns is much less common and it can often be omitted, because plurality can often be indicated by use of plural classifiers (e,g. - a group of; a class of) or plural adjective (e.g. - those; these).
們 is always used on people or other situations when you want to express personification.
If we write 一群學生 (a group of students) or 那些學生 (those students) we no longer need 們 to indicate plurality. We know it is not a single student.
Adding 們 and write 一群學生們 or 那些學生們 would be redundant.