"Nosotras" and "Nosotros" are exactly the same word, but the first one is a femenine word, and the second one is a masculine word. Therefore, when you are with other people, and among the people there are men and women, and you (or some of them) want to talk about "we" in spanish, you (or that another person) has to say "nosotros" ("nosotros" can we used by men and women indistinctly). When you are with other people and everyone are women, you (or another person there) has to use "nosotras". And the final situation is when there are men at the same place and they want to use the word "we" in spanish. They must to use "nosotros", because that is the masculine form or the word. So, remember this:
"men + women" using the word "we" in spanish: Nosotros
women using the word "we" in spanish: Nosotras
Men using the word "we" in spanish: Nostoros
I hope I'd helped you!! ^o^