Edie McClurg's character...

I think she's part of the group because Chris is seen being friendly with her in the beginning and she tries to get her to defend her against the Coach. Also, she's seen hanging out with Norma near the end.

Though, she might just be a hanger on. It's possible she's, I dunno, someone whose only friends with them because she's their designated driver when everyone else gets drunk or she does their homework for them or something. Considering Norma and 2 other girls are seen bullying her at the beginning, that might be the case.