I think it is thoughtful to both ask your wedding party to be in your wedding party and thank them for being in your wedding party in a sweet and personal card.  It is a nice memento for them to save and shows you put thought and effort into the asking and thanking.  Here are some of my favorite wedding party cards:

From Tallu-lah
There are also some really cute bridesmaid and groomsmen cards from the Wedding Chicks, and they are free printables!
Go here for the groomsmen card, here for the best man card, and here for the bridesmaid card.