Easy to install. Clear installation guide with clear instructions is included. Any webmaster with minimum experience can install the script easily.
Security measures to avoid spam or abuse are included. The invitations are not sent out unless the user enteres the security image seen on the page. This ensures that a human is using the script and not some hacker bot.
Customization header and footer so that you can easily merge the script in your site design.
MySQL database backend is used to track invitations and how many of those are really acted upon. In other words you can keep track of who sent out invitations, how many invitations were sent out and whether the invitees really clicked on the invitation link and visited your site or not. If you know PHP and MySQL you can use it to further enhance the script in countless ways.
Your users themselves can check what happened to the invitations they sent out. They can send reminder invites if their invitees have not acted upon the invitations or may be have not received them.
Demo:
If you want to see the demo of the script, you are welcome to see it in action on one of my sites -