。
She's very popular among boys, too.
Literal
She [topic-は] boys [genitive-の] interval [in-で][also-も] very popular [subject-が] exists.
の間で is the standard way to say 'among (a group)' — literally 'in the interval of.' The location particle で marks the locus of the popularity. でも here is で+も ('also at/in'), implying 'among other places too — even among boys.' 人気がある ('is popular') is the standard collocation: 人気 ('popularity') as the が-marked thing that exists, with ある as the predicate. There's no native verb 'to be popular' — Japanese uses this 'popularity exists' construction instead.