The article goes into detail; it's worth the read to answer your questions.

From my experience, there is very little process and oversight being followed. I had my number ported over (with my knowledge) to Tmobile by a 3rd party, however Tmobile had not attempted to verify my consent. The store associate took this person at her word. My then current phone stopping working caught me by surprise.

I can imagine if I signed up for a family plan, any store associate would be happy to move any number of phone numbers into my control.