。
She is a member of the tennis team.
Literal
She [topic-は] tennis-club [to-に] is-affiliated.
所属する (shozoku-suru, 'to be affiliated with, to belong to') is the most formal of the membership verbs — used on official rosters, sports introductions, business cards. Stronger institutional flavor than 属する: where 属する is generic 'belong,' 所属 ties to a specific organizational unit. The Sino-Japanese compound 所属 ('place + belonging') reinforces the institutional framing. In spoken Japanese, テニス部です ('She's [in the] tennis club') is far more natural; this sentence reads like it might come from a sports broadcast or formal introduction.