Invitations that are sent via an eCard will display the view statistics on the Sent Cards screen. (eCards > Sent Cards). Those statistics will also show you who has responded to your invitation.
When you are on the Invitation List for a particular event, (Invitations > Manage Events: select ‘Invitation List’ in the drop-down list next to the event) you will see the total number of people who viewed you eCard, and how many people responded. If you scroll down through the list of individual invitees, you will see icons showing whether that person viewed the eCard and if they responded.
Tip: From this screen, you can also create reminder eCards automatically addressed to ‘People that did not view the eCard’, ‘People who did not respond’ or ‘All people’ on the invitation list.