Well, you could send something really fun in the mail...
Super Cheesy: send a little note like from 3rd grade that asks them to be your bridemaid then ask them to check yes or no
Or: you could send a little package with pics of each of them with you and perhaps a fun story from your past and maybe a little fun trinket and ask them.
I don't know, I wish I were more creative...but I bet some of these gals will have some fun ideas.