朋友们 friends 两个朋友 2 friends. You already give a number so we know there are many friends. If you don’t give a number you need to put 们 so we know it’s more than one friend

- Carolann Nashipae

们 is used when clarity is needed. In the phrase 两个人, you don't need clarity that you're talking about multiple people, so you don't use 们.

Furthermore, the image I get in my head when I look at 两个人们 would be two groups of people (it's still not grammatical, though).

- Trevor Kafka

Only if the noun means a person *and* the plural is not expressed otherwise (say, through ""兩个"; additionally, mostly in forms of address and rarely otherwise.

- Magdalena Jarczyk

You said两个人 What you have expressed is a group, and there are two people in this group! 两个人们 But there is no such usage in Chinese grammar! At least I won't use it as a Chinese!

- Chou Helen

You use 们 when you don’t have other modifiers. In cases such as 三个人，四个人，you have more accurate “三个” “四个”, already, so adding 们 become redundant.

But there’s rare exceptions, 我们俩，us two, which is equivalent to 我俩。But you would never say 两个人们，三个人们。

Sorry I’m not a Chinese teacher, so might not have the best explanation. Just listen, listen, listen and then read. You’ll figure out the grammar naturally.

- Tony Iron Chen