。
She is a member of this organization.
Literal
She [topic-は] this organization [genitive-の] member is.
である is the literary/formal copula, the written cousin of だ. It carries a weight that だ doesn't — appropriate for declarations, formal definitions, news writing, or essays. In casual or conversational contexts, you'd hear 会員だ instead. 団体 ('organization, group, body') and 会員 ('member, registered participant') are both Sino-Japanese formal nouns appropriate to this level. The whole sentence reads like a dossier entry or a profile snippet rather than a casual remark — exactly the register where である earns its keep.