You are right, because "men" is plural, it is already properly differentiated, but that is not the point. "You all" is (sometimes) used in English to differentiate singular and plural second persons. As long as the phrase is "you all are men" (with that word order), it is simply more specific than "you are men." However, "all" can change the meaning depending on placement and other words in the phrase (e.g. "You are a man" is not the same as "you are all man" and "you all are men" is slightly different from "you are all men").
I almost answered the exercise with "Y'all are men," still in common usage where I come from. I leave it to you'se guys to decide if you want to do the work to include regional differences in possible exercise answers. "I hear tell" that there are at least 11, maybe more, accents of English in the UK alone! Good luck with rhyming slang.
I am confused. In the "to be" conjugation chart I have, it translates εσεισ as "we" and εισαστε as the "you" form of "to be". It says εμεισ ειμαστε is "we are" and εσεισ ειστε is "you are". (I know my ending σ is incorrect - I haven't figured out how to change it on my keyboard yet; and I don't have my accents figured out; I'm new.) Can someone please explain?