Being an authorized user does not help their credit score - at least the last time I checked.
I believe they have to be co-owners on the card with you. Therefore, you'd be linking your credit history to theirs. If they have none, they get the benefit from you, if you have a good score. If they have a bad score, it will impact you negatively.
It also hurts you if they go on a shopping spree and don't pay up
