Here's a great AU letter than I ran across. I've used this successfully to get added as an AU on two accounts.
An AMEX with a 20 year history and a JCPenny's card with a 3 year history. Both have excellent payment histories.
Dear [Friend/Relative Name]
I am working on improving my credit score. Two factors that are preventing me from having a higher score are that I have too few positive accounts and/or that my accounts are too new. In order to address this issue I’ve been looking for any friends or relatives with good credit who might be willing to help.
I’m asking that you add me to one or more of your credit cards as an authorized user. There is no need to have a card issued to me nor do I even need to know the credit card number. In doing this, there is no risk to you since I’ll have no way of using your card. Even on my credit report the credit card number will be truncated so that I will never know the complete account number nor will I have the expiration date or the CVV code, both of which are usually necessary in order to make a charge.
Doing this favor for me will allow your good credit to show up on my credit report and should cause my credit score to increase. Generally this procedure can have a tremendous effect on a person’s credit score, raising it by as many as 100 points. In order to positively influence my credit, the account(s) should have excellent credit history, have very low balances (less than 20% of the credit limit) and, ideally, they should have a credit history of at least three years.
If you have any reservations about this procedure, please let me know so that I can address your concerns. I truly appreciate your assistance, and I’m confident that you are doing me a great service.