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It will come up naturally as you progress through the story. You will literally be given a choice to "invite to live with you"

To be more specific.
You get two from progressing the fey branch of the main story.
Make sure to pick the right choice.

Also you get one from Roxy's story.

You can also invite Jessica's friends, provided you saw their second event.
Laura appears in the living room.
Lucille likes to shop at night.
Renee leaves a message for you in Jessica's office.

After completing the second event you can talk to Jessica in the kitchen and ask her to invite her friends.