。
She's the treasurer of the club.
Literal
She [topic-は] club [of-の] treasurer [object-を] is-doing.
会計係 ('treasurer, accounts officer') — 会計 ('accounting, billing') + 係 ('person in charge of'). The suffix ~係 is widely used to designate someone responsible for a specific role or task within a group: 案内係 (guide), 受付係 (receptionist), 風紀係 (discipline officer in schools). ~をしています ('is doing X') is the everyday way to say 'currently holds the role of X' — present tense ~ている for an ongoing state. In school clubs, these positional roles (会長, 副会長, 会計, 書記) are formally rotated and provide early lessons in organizational responsibility.