。
Is she on the tennis team?
Literal
She [topic-は] tennis-club [in-に] is-in [question-か]?
入っています (here contracted to 入ってます with the い-elision typical of casual polite speech) literally 'is in,' but for clubs and groups it functions as 'is a member of.' The に marks the club as the location of membership. Note 入ってます: the て-form of 入る followed by います, then the い elides — the polite ending stays, but the spoken form drops a syllable. This kind of casual contraction is universal in conversational ですます-form Japanese, and a hallmark of natural-sounding speech.