What the heck?! Thats just weird. Have they booked this trip now? You dont have to invite them to the wedding.. they can still come and hang out with your friend and everyone but the actual wedding day will still be your wedding day and they wont be involved. At least thats what I would do. I wouldnt pay for three more guests that I wasnt even friends with, or that i didnt invite to the wedding. But I would be ok with other people coming and being at the resort since my wedding is only one day of the week.
So I guess I would just tell my friend that thats great that her friends are going too but that they wont be coming to your wedding and that you still want her to be a big help and part of your day, since she is a bridesmaid.