You have probably looked at Duo's translation. There is a verb, juntar, which is not the adjective junto (together). Since they can take feminine and plural forms then you get juntas. Juntas can also be the second person present of juntar.
It is true that if we had to translate "We work together" from English we would choose "juntos" or "juntas" according to the gender of the group, but here, because we are given the Portuguese sentence to translate, we know "nós" is a group of females. [Edit: Not sure why this simple statement of fact has been downvoted!]
"Nós" is a Personal Pronoun and does not indicate gender in Portuguese. I know that in Spanish there is difference between "nosotros" and "nosotras", BUT in Portuguese it's the same word for male and female: NÓS
The point about Spanish is very interesting. However, even though "nós" is gender neutral in Portuguese and the group it represents can be all male, all female or mixed, it is sometimes possible to infer which it is. For example, a statement like "Nós somos bonitas" could only be made by a group of women.
Yes, but if the program gives you the English sentence both should be accepted.
Aham... that's right! Curiously if there is nine pretty girls and just one beautiful boy, they necessarily must to say "Nós somos bonitos". In other words the male gender prevails. Another exemple: ninety-nine girls and one boy will travel: "eles vão viajar". Legal conversar contigo Davu. Abraço!
Occasionally this male dominance is subverted. I've quoted this little rhyme from Arnaldo Antunes before:
Neto e neta são netos, no masculino.
Filho e filha são filhos, no masculino.
Pai e mãe são pais, no masculino.
Avô e avó são avós
Of course, it is "os avós" but that would spoil things. :-)
Regarding your latest comment: perfect -- "avós" it's an exception. Congrats!!! You're good! :-)