add contacts to facetime

Your contacts are added via your Contacts App:

Add and use contact information on iPad - Apple Support

FaceTime calls, using the FaceTime App, are available between users of Apple devices.

Contacts that are Users of Apple devices, that have enabled FaceTime, will be automatically contactable using FaceTime; such users will be identified on the individual contact card. Non Apple-users, or Apple-users that have not registered for (or enabled) FaceTime will not contactable via FaceTime; the corresponding contact card will not show FaceTime as a calling option.

Make and receive FaceTime calls on iPad - Apple Support