。
She's popular with the other girls.
Literal
She [topic-は] other [genitive-の] girls [in-に] popularity [subject-が] exists.
に as the population among whom she is popular — ~に人気がある is a standard frame for 'popular with X.' Note the 'with' here is に, not で or と; the popularity is conceptualized as existing within the target group. 女の子達 takes the plural marker 達 because the speaker wants to specifically pluralize 'girls' as a group; 子供たち, 友達, 女性たち all work the same way.