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With Find My iPhone, you can locate your devices of others' devices that are associated with your account. Thus you can get your iPhone or other devices back easily or if you can't get them back, you can protect your private information by remotely erasing the data on the lost device. Find My iPhone is really useful to protect your device and your information, that's why many people are looking for the way to add a device to Find My iPhone. The ways to make it are shown below.
Step 1: Go to "Settings" on your device.
Step 2: In iOS 10.3 or later, tap on your name and choose "iCloud". In iOS 10.2 or earlier, select "iCloud".
Step 3: Turn on "Find My iPhone".
Step 1: Click Apple menu (the Apple logo) and select "System Preferences".
Step 2: Choose "iCloud".
Step 3: Scroll down to find the "Find My Mac" option and click the checkbox next to it. (If asked, enter your admin password.)
Step 1: Go to iCloud.com and sign in with your Apple ID.
Step 2: Select "Find iPhone" among the options.
Step 3: Click "All Devices" in the top middle of the screen and choose the device you want to remove.
Step 4: Click "Remove from Account" to remove a device from Find My iPhone.