Is a girl who has no uterus considered intersex? Is it permissible for her to sit with girls? Will she be rewarded for her patience?

You should praise Allah for the soundness of your creation, be patient with His decree, and ask Him for righteous offspring and a happy marriage. You are not an intersex person, so there is no harm in mixing with female colleagues, and you can marry if sexual intercourse is possible, while secretly informing your husband of your condition. You are not required to tell everyone your truth, and you should seek refuge in Allah from people's criticism. Occupy yourself with beneficial knowledge and the biographies of the righteous predecessors.