Q: How do I add my boyfriend’s iPhone to my devices under Find My iPhone?
A: Apple’s Find My iPhone feature is only designed to be used with devices that are configured under a single iCloud account—essentially for your own personal or family devices. This means you can’t actually add your boyfriend’s iPhone unless he has signed in to Find My iPhone with your iCloud Apple ID and password on his iPhone, or unless you sign into the Find My iPhone app with his iCloud Apple ID and password.
Note that you can enable Find My iPhone on a secondary iCloud account under iOS 6, which may be a solution if you really want to share a keep both of your iPhones under a single Apple ID for the Find My iPhone service. However, in iOS 7—scheduled to be released next week—Find My iPhone will only be supported as part of the primary iCloud account.
If you’re simply interested in keeping track of each other’s locations while keeping your own separate Apple IDs and passwords, you may want to take a look at Apple’s Find My Friends app, which is designed expressly for the purpose of sharing your location between different users. This app also provides additional features such as allowing you to receive alerts when friends arrive at or leave from specific locations.