。
She belongs to the tennis team.
Literal
She [topic-は] tennis-club [to-に] belongs.
属する (zoku-suru) is the formal/literary verb for 'belong to, be part of.' Takes its target with に. The casual everyday equivalent for clubs would be 入っている ('is in'); 属する is more often heard in formal contexts — political parties, organizations, academic departments. For school clubs specifically, the most natural choice is 入っている; 属する here gives the sentence a slightly stiff, almost biographical-profile feel, as if reading from a CV.