Chloe's you get by progressing the main story branch of the organisation, then talk to her in her room during the morning.
Gwen, don't have sex with her for a month, but also you need to leave the yandere mode turned on in preferences.
Jessica's appear in her story, just pick the right choice.
Pixie's, one if you bang her on her "regular size" the other if you give a class in the college during the weekends.
Qarinah's is after finishing the story and you go to sleep.