Their secret sauce is that they have spent a lot of time and money innovating on how to not kill battery life. The app polls users for location on different intervals. Periodically if no one has the app open, but if someone opens the app it will start polling people whose locations are available to them. Find My Friends has a similar approach, although it ONLY polls when someone has Find My Friends open that has a shared location available, and it doesn't store the data (last I checked).