No credit card required
Browse credit cards from a variety of issuers to see if there's a better card for you.
It depends upon whether the authorized user is granted use of the account, such as a card.
The principal card holder is obligated, and any use he makes will be reported to the account, affecting your score, and then indirectly, his.
If the authorized user is not granted use of the account, then your use will affect his score, but not visa-versa.
As for possible affect on the AU, assuming your history results in a score gain for him, that is all gravy provided a potential creditor accepts the fact of his scoring being based on credit history that is not his. If a potential creditor bases their determination primarily or solely on a FICO score, then the AU will help. That is the case with many lower-principal types of new credit.
However, if he should app for credit wherein the creditor does a more detailed review of his CR, they may balk at accepting his score when it is based on the history of another.
Having no way to back out your history from his score, it might lessen the value of the score to their determination. Creditor decision that is hard to predict.
It only hurts if the bill doesn't get paid! I added my son as an AU to my oldest, perfectly paid card right before he turned 18. It's helped his score greatly and hasn't hurt me at all.
Right after my son turned 18, we tried to pull his score and his file was too thin even though I'd added him to the one card. So, I added him to my Macy's account (a couple years old, low balance and also perfectly paid) and was then able to pull a score - which came out at 732 - higher than mine, the little brat! Then he was able to get an account on his own and is establishing himself but I'll leave him on my cards forever, just to get the longer average age of accounts.
I added myself to my husbands credit card and it helped my score and didnt hurt his. I think the only thing that hurts is if the utilization is high or there are missed payments. If that happens it will show up on the AU"s report as well.