I used to have an unsecured card with bank of america for which I had added my wife as an authorized user and I remember that my account was showing on her credit report. I’m not sure wether or not it was counting towards her credit score as a card of her own, but it was there so I suppose it was counting for something.
Just make sure that you get someone with a great credit history who’s unlikely to default. I’m sure my wife’s score took a hit when I defaulted and my card was closed…