Head to Kyle's place to update this quest. After the scene, some new sub-quests will become available. Kyle and Cartman can be set as buddies now. Depending on the who you sided with the last time, either Cartman or Kyle will be added to your Friends List at this point. Don't forget to talk to Chris Donnely as well to add him as your friend.
To complete this quest, you have to get the girls to join your group. Refer to the "Recruit the Girls" page for more details.