I'm helping a younger member of my family build a credit history by adding them as an authorized user to my Amex Platinum (free Gold card). I'll hold on to the card.

I've researched this a bit, and it doesn't seem like this will actually help their credit that much because it is a charge card and no credit limit is reported to the bureaus so she won't get the benefit of my low utilization (nor do I, I guess). Does anyone know for sure? I can add them to a Visa/MC but I have a 5k MR/AU promo available currently so I'd like the points.

Thanks