The Bridesmaid dress needs to be bustled, for them to move freely in the wedding party without the dress getting ruined. Bustling is picking the train of the dress off the floor and securing it to the dress and not drag along dirty floors.

Try on your dress and the shoes you’re wearing at the wedding to design a bustle of an appropriate length. Ask a friend to Pick the center of the dress’ train and Place two pins, one in the middle and one in the back of the dress.
Sew a button and loop exactly where the pin sat with a thread of same color as that of the dress; else it will be noticeable. You can either buy bustle loops or create your own

Try on the dress with buttoning up the dress’ train to ensure the length is appropriate. If not, then make adjustments and re-sew it to the proper location