Unable to add people in Find My Friends

The option to share your location with others and vice versa in Find My is called “Share My Location”, which is present on your screen. Once you tap that and add a contact, you can choose how long you want to share your location and request them to share back.

This is how Find My works for Friends. Share your location in Find My on iPhone - Apple Support

that would be too easy to track people, if this would be possible!

If you want to track another person, it’s only possible if the other person decides to share his/her location with you