It is possible to track others' iPhones in certain instances. As of 2016, software like TeenSafe claims to help parents keep track of teens' GPS locations. Also, apps for the iPhone like GPS Tracker and Phone Tracker claim to allow tracking location of friends and family.
Apps including GPS Tracker and Phone Tracker are permission-based, so they can only be used if the owner of the other iPhone being tracked allows tracking. Some users complain the tracking is not always reliable or the apps charge upgrade fees to continue to be used, but overall these apps have ratings of greater than three out of five stars.
TeenSafe operates on a PC platform to track locations. It requires a monthly fee and requires phone contents to be backed up to iCloud to monitor phone contents. Some individuals give it positive reviews, while others complain monitoring does not always work.