。
She served as a bridesmaid at the wedding.
Literal
She [topic-は] wedding [at-で] bride [possessive-の] attendant role [object-を] served-as.
花嫁 means specifically 'bride' (the just-married or about-to-marry woman) — distinct from 妻 ('wife,' general married woman) and 嫁 ('young wife / daughter-in-law,' often colloquial). 付き添い役 ('attendant role') splits into 付き添い ('attendance, companionship') + 役 ('role, part'), and 役をつとめる is the standard collocation for 'to perform / fill / play the role of.' The whole concept of a bridesmaid is a Western import; traditional Japanese weddings have no direct equivalent — they instead use 仲人 ('matchmaker / go-between'), an older couple who formally supervise the union.