I've got a nice program which does beautiful cards, invites, etc. I've never tried to send one by email as I like printing them out and mailing them. It is Greetings Workshop. You can create anything you want with it.
Signing guest lists, etc. is a bit more complicated. What about a private group at Yahoo? That's what we did when we wanted to keep Art Thiemes' computer project a secret from him. Only those folks to whom you send an invitation are allowed to post, etc. It was very easy to start the group. Also, people can choose whether to be notified by email when someone else posts to it, etc.