Group GPS (free) is one of those "Gee, I wish I had thought of this" apps. To get started, you sign up for a free account by supplying a user name and an email address. Create an activity, like 'bike race' or 'desert hike,' and send an invitation to your friends. If they don't have Group GPS, they get a text message with a download link. When everyone responds, you'll see each person identified on a map, and their movements update as they move around. I can think of lots of applications, like meeting friends at a location and seeing that they are on the way, finding people at a concert, car rallies, almost anything.
When the activity is done, just stop Group GPS and you're removed from the activity. You can also pause the app anytime and return. The map can be zoomed in for more detail on a single position, or zoomed out to see everyone else's location.
The Group GPS account was easy to sign up for. It doesn't matter where the other people are; even if they are in another state, you'll see the map stitch to fit everyone, and you can watch your group converge. Apps like Glympse are perfect for letting people know where you are and your ETA. Group GPS lets everyone in a group know where everyone else is. In testing the app, I noted that the position updates aren't in real time, more like once a minute or so. This was likely done by the developer to preserve battery life, although I'd like to see an option to step up the update frequency. The app is ad-supported, but the ads are not invasive or distracting. Having said that, I'd be glad to see a paid version with no ads and a few new features, like being able to message the group from within the app.
If you'd like to see the app in action, you can check out a video on the developer website. A similar free app is FastWhere, which also does location sharing.
Group GPS is a really great idea, and I think most people will find some good uses for it. The app requires iOS 7.1 or later. It's not universal, but it is optimized for the iPhone 5.