You should ask people to be in our bridal party about 6 months before the wedding.
You should register when a gift-giving event is planned. So if someone offers to host a shower, you should register before the shower invitations go out. If not, then register before the invitations go out for your wedding. You don't want to register too early, because a) no one is going to be buying presents then, anyway, and b) things will go out of stock between setting up the registry and people going to buy things off of it.