First and foremost, decide if the person in question will be a good fit for your wedding? Most bridesmaids are close friends and will enjoy sharing this special time. I think finding a fun way to ask is best, making it a special and memorable experience. Being in person is best. Find a fun small trinket gift that you can enclose the message in. Or take them with you shopping, strolling into a bridal shop and simply ask them, "Would this be something you would like to wear at my wedding?". It is a special time, surround yourself with special friends to share it with. Be clear on your expectations as well. Communicate on whom is paying for the dresses, time frame of the wedding, ect. Having this laid out when you ask will help with the end result. You don't want a Bridzilla ending! Good luck!