This is an interesting question. there are at least two meanings for "大家", one means "we/us", and the other means a well-know person in a field.
so, if "大家" means "we/us", you can't say "大家们", it's not correct, because in this case "大家" has been represented "we/us"; but when you describe a group of people who are well known in a field, such as calligraphers, you can say "大家们", here are the examples
English: The calligraphers from china have been sleeping.
but most of time we would like to say in chinese as
in most case of chinese, adding the word "们" at the end of a creature may means the group of this kind of creature. "人们" means people, as "人" means human being.
However, it can't be described as a simple question, to understand all about that you may need to know more about china and chinese.