。
She's on the basketball team.
Literal
She [topic-は] basketball-club [genitive-の] one-member is.
部 ('club') is the standard suffix for a Japanese school's extracurricular sports or culture team — テニス部 (tennis club), 吹奏楽部 (brass band club), and so on. Membership in 部活 ('club activities') is a defining part of student life from middle school through university; clubs typically practise every weekday after school and most weekends, and many students identify strongly with their team. The phrase ~の一員 ('one member of') is a stock noun-phrase predicate for declaring group membership, slightly more formal-sounding than ~のメンバー (the loanword equivalent).