If you want to add a second person to your iPhone's face recognition ID, you can now do it with iOS 12 as well.
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Apple calls this feature an "alternative look", so technically it is aimed at users who may wear different hats for their work the day, such as safety equipment. However, you can use it to add someone else to the repertoire of your iPhone's Face ID.
First open the app settings.
Scroll down and select "Face ID and passcode."
Enter your passcode to continue.
Then tap Set Up Alternative Appearance.
Tap "Start Below".
The person you are adding goes through the facial recognition scan, as you did when you first set up your iPhone. 
When you're done, click on "Done" below.
And that's all! The second person you've added can put your phone right away with her face Of course, even without a face added to Face ID, they can enter their passcode if they know it.