Answer:
It means that Denise and her friends psychologically identify with each other.
Explanation:
Psychology and sociology explain that an "In-group" is a group of people who share thoughts and ideologies and therefore the people who make up this group are psychologically connected, that is, they identify themselves psychologically with each other.
This is different from an "Out-group" which is a group of people who do not identify psychologically with each other.
