Their appearance will also evidently change over time, though it's not clear whether that's determined simply by level, by equipment, by skills chosen or some combination of all three. Earlier builds of the game suggested you could hire more than one follower, but this glimpse shows only one at a time.
The original poster of the video on YouTube reports that Blizzard has now "officially released" it after confirming the leak, but there's no sign of it on the official website yet, and we haven't heard anything official. If we do, we'll let you know.
Update: Here's the official page. Looks like we were right: only one at a time, though each follower has their own story that "entwines" with yours as you play the game. Sounds fun!