I'd like to know too. Maybe this feature is disabled?

this feature was not disabled for me, you can select anyone as a friend or as a family member (some pics may be better if only family see them...) this is how
you see by a persons comments thier name, click on that, and you will go to thier home page. When you get there you will see the word profile when you click there you will see follow person click that and you are a contact!

As a contact there will appear following, click that and you will see edit relationship, when you click that you will be able to mark a person as friend or family, as well as pick how you can follow them (I for one don't have mobile, so don't allow that option.)
DONE try it out and let me know!