One thing that's important to remember is that the first photographer has to leave room on the sides for both photographers to get into the shot. You'll notice in one of my shots I'm clipped off on the left side, and Lynn La, the second photographer, had no choice but to line up the shot the same way. It takes some getting used to, but make sure to leave space for everyone in the photo.
Like many photography apps, Groopic also lets you pick from a number of different filters to give your group shot a little extra pizazz. When you're finished, you can share your group picture to Facebook or Twitter, send via SMS or e-mail, or simply save it to your Camera Roll.
The only confusing thing I noticed about Groopic is that it can take some time to snap the photo after you press the button. It's something you can get used to, but it would be nice if the photo snapped immediately when pressing the button. It will be something you'll need to point out to your second photographer to make sure the photos align properly.
What's unintentionally great about the app, though, is that you can get creative with how you take your group shots to get funny results. In my testing, I had the same photographer take both shots, while I switched sides in the group to make it appear that my twin was on the other side of the photo. There are probably other ways to use the app depending on your creativity. You could have the same couple on each side of the photo doing different poses, for example, and have the people in the middle reacting to their poses.
While there may be other apps (in more than 900,000) in the App Store that blend two images into one group photo, I've never seen one that's quite as easy to use as Groopic.