Like other attendants at a rehearsal dinner, bridesmaids should wear clothing appropriate for the tone set for the event. When formal invitations for the rehearsal dinner are sent or the dinner is at a formal location, dress should match. Casual attire is appropriate for a more casual dinner.
A tip to choose the best outfit for a rehearsal dinner is to research the venue first and dress appropriately for that location. A simple dress is usually appropriate for most rehearsal dinners, unless the wedding is a very casual event where jeans are welcome. All wedding attendants, especially the bridesmaids, are expected to attend the rehearsal dinner as well as any other pre-wedding events.