be one of the girls
To be an accepted member of a particular social group, especially one made up primarily or entirely of females (usually women, not girls). Jen's nice and all, but she's just not one of the girls.
be one of the ˈlads/ˈboys/ˈgirls (informal) be a member of a group of friends of the same sex and a similar age, who meet regularly to enjoy themselves: His wife doesn’t understand that he likes being one of the lads from time to time. ♢ She’s never really been one of the girls. She much prefers the company of men.