If you've already sent a card you created to the recipient but decide that you need to add more videos then you can.

As the card creator simply open the card in your dashboard (click on the card in your Created tab). On that screen you'll see a button 'Add a video'. You can add another video.

Or, you can click on the 'Invite others' button to get the contributors link. You can send that to anyone who has not already made a contribution. 

Keep in mind that as the card creator you can add multiple videos to a card. However, contributors can only add 1 video to a card they were invited to contribute to.