Since the idea of female groomsman is relatively new, there aren't set rules about what to wear. According to Generation Tux, many women who stand up next to the groom during the wedding choose to rock a suit, specifically something more fitted. This allows them to blend seamlessly with male groomsmen while still looking chic. 

Women who may not want to wear a tux should consider the color palette of the wedding when choosing their dress. Brides recommends choosing a dress that matches the bridesmaids, but in a darker shade to stand out a bit more. They also recommend matching your dress to the groomsmen's suits, so if they are all wearing black tuxes, consider a black gown, regardless of the bridesmaid's dresses.

Overall, you want to feel comfortable and beautiful no matter what your title is on the big day, but you also need to respect the bride and groom. Speak with the couple to get a feel for the overall look they are hoping to achieve.