At lunch some of them share a Diet Coke.
Unnamed Girls Who Don't Eat Anything are a clique at North Shore High School as defined by Janis Ian. As their name says, these girls are very slim and, unlike other students, they don't eat absolutely anything in the cafeteria.
None of the members of this clique had their names revealed. However, judging from the number of students who were sitting at their table while Janis was explaining 'the map of the cafeteria' to Cady, there are presumably only four members in the group. She simply wrote "Thin Girls".