The words "vosotros" and "vosotras" mean "you" in english. The only difference between them is that "vosotras" is used specifically for women and "vosotros" for men. Now, if you are in front of a group with either men or women, you have to use "vosotros" because in this case you have (as I said before) men and women together.
To demonstrate what I said, here are some examples:
Vosotras tienen que hacer la tarea. ( You have to do the homework) -> Because I used "vosotras," the sentence is talking about women.
Vosotros hicieron un buen trabajo en equipo. (You did a good team work) -> here the sentence is talking either about only men or men and women. That depends on the situation.