So whether or not it is masculine or feminine would be determined by whether the person speaking is referring to "us" as women or "us" as men? What if it "us" refers both men and women?
If it is a mixed group that contains both masculine and feminine words or men and women, masculine endings take precedence.
How do we know what gender to use?
Since gender is not indicated in the English sentence, translations using all genders are accepted.
Whats the difference between "my jsme" (we are) and just "jsme" (we are)? How do I know when to use "my" and when not to?
Not much difference, the pronoun is optional. It can should be used when it is stressed but can be used at any time.