Why is 你们爱猫 translated to "you love cats" (and not e.g. "you all love cats")? I've been using HelloChinese and one of the quizzes it gave (without much explanation) was something like ni men ai mao ”你们 爱 猫“ to mean "you love cats". However, reading it I thought it was "you all love cats" (like the southern US slang Y'all). In google Translate it seems to mean the same thing with or without 们

What exactly does men add to the meaning of this phrase and where should it be used?
http://www.cantonese.sheik.co.uk/dictionary/characters/5/
  
  們:
  
  an adjunct to a pronoun or noun to indicate plurality


Pronoun:

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)

Noun:

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. 

For example: 


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.