Head to Kyle's house for a scene. After that, Cartman and Kyle will be unlocked as buddies. Cartman will also become your friend. When ready, head to the Town Hall and talk to Annie.
She'll take you to the girls' ongoing meeting. Your objective will be updated and a new sub-quest will be added. (Pose as Bebe's Boyfriend)
This will be complete once you’ve successfully recruited the girls to your side. Refer to the “Recruit the Girls” page for more details about the sub-quests involved.