From the My Events list, click on the title of the event.  Then, select Track Status of Invitations from the left hand feature menu on the Manage event page.

Track the invitations you've sent - the recipient's email address, name, the status of the invitation (Opened, Clicked, Responded, Not Responded), and corresponding notes.

You may view ALL invited guests by clicking the "View all invited guests" link towards the bottom of the page.

You can also resend the invitation to all recipients or a subset of recipients (e.g. those who haven't responded) by first using the "Filter Invites" drop down and then using the "Resend Invites to..." button.

You can also track invitations that you have scheduled to send at a future date.  You can preview, edit, copy, and delete those invitations.

Click here to learn more about tracking RSVP responses.