It's easy to add a message or photo on the back of someone else's card, and it's a nice way to collaborate in making the box.
First, go to the group page for the AwesomeBox by logging in and looking at the drop-down menu in top-right corner of browser.
Second, scroll over a card you want to collaborate on, and you will see it flip to the other side that looks like this:
You'll get to a field where you can add a different signature line than was on the front of the card.
If there is already a photo, you'll see a "Add Message" button instead.