With GPSTracker you can track all your iPhones. Do you have a transportation company and want to know, where your drivers are at any time? With GPSTracker you are able to see all your iPhones on a map.
The app supports multitasking (so tracking, whilst running in the background is possible)
Continued use of GPS running in the background can dramatically decrease battery life.
The only thing you need besides the app is a free account at http://www.harzig.com/tracker