。
She served as the leader of the club.
Literal
She [topic-は] club [of-の] leader [object-を] served-as.
つとめる (written 務める when about fulfilling a role) takes を for the role being filled — 'serve as X.' This particular kanji 務 is for serving in a duty or role; the homophone 勤める (also tsutomeru) is for being employed at a place (会社に勤める, 'work at a company'), with に instead of を. Tracking which kanji-particle pair to use is part of mastering this verb. 指導者 ('leader, instructor, guide') decomposes as 指導 ('guidance, instruction') + 者 (person), commonly used for the head or main organizer of a group, club, or movement.