。
She plays an important role in our organization.
Literal
She [topic-は] our group [in-では] important role [object-を] is-fulfilling.
役を務める ('to fulfill a role') is the standard collocation for occupying a position or playing a role — both literally (in a play) and figuratively (in an organization). 務める ('to serve, to fulfill duty') is the verb of filling an office or a duty; compare with 演じる ('to perform, to act'), which is more about portrayal. 団体では uses では (the contrastive/topical で + は) — the location 団体で ('in the organization') is foregrounded as the relevant context. The ~ている marks ongoing engagement: she currently holds the role.