Sarah Grossman is a member of the CCP clique. She was known to be MacKenzie's friend, but did not end up with her in book 2. Although, she did support MacKenzie when she was mean to Nikki.
Very little is known about Sarah, as she never spoke in the book, but from seeing she used to be from MacKenzie's little group, she could've been snobby, mean and partially spoiled. But she is especially a phony. When Sarah and Lisa Wang got birthday invites for Mac's birthday, they both cried phony crocodile tears of joy.
In book 1, she was sketched by Nikki. She had cat-eye glasses and had shoulder long, curly hair. Since she was member of the CCPs, she wore designer outfits.
It is known that MacKenzie was her friend, until MacKenzie went to Jessica. Sarah, Lisa Wang and MacKenzie were a trio, and Nikki described them as the "fashion police". Now, nothing is mentioned about Sarah, but an image in book 3 of Mac's Maniacs shows a girl with cat-eye glasses on her head and had a bun with curly hair. So she could've been performing with MacKenzie. In the ballet class, MacKenzie, Sarah and Lisa did a routine together and finished their dance with (as described by Nikki) "cheesing".
Nothing was ever mentioned about Sarah's status. But since she is a CCP, it could be a theory that she liked one of the jocks or popular guys.