Do joint credit cards help build good credit?
When another person adds you to a credit card as an authorized user, the credit card company places the account on your credit report as well. Often, the account will carry a note indicating that you are an authorized user rather than the primary cardholder. Even so, this serves to substantially improve your credit history. On the other hand, the account will not typically show up with the entire account history, but will show only from the time you were added as an authorized user.
Note that if the account goes delinquent, it may negatively effect your credit report and the credit card company may even attempt to recover payment from the authorized user.