Simply sign into your account and click on the drop-down menu in the top right hand corner that has your name within it. Click on "Events" and click on "Past" on the next screen. Below the events you've hosted, there will be a "Copy Invitation" link.
After clicking this link, any information you originally entered in the fields provided and your entire guest list will automatically transfer into a new invitation. You can then edit your invitation's information, design template and guest list before sending it out to your guests.