Something I read on a credit forum is that if you get added as an authorized user to someone else's existing account, that account goes onto your credit report, and if it has been open long enough and is in good standing then your credit score goes up as if it was one of your cards.
Now, after reading too many horror stories of people whose friends have gotten them in trouble by misusing credit in their name, this authorized-user idea is something I would recommend that people only do with friends they REALLY trust. And maybe as added the security, the real cardholder should keep/cut up the additional card that the credit card company sends for the authorized user. This way, the authorized user is prevented from actually using the credit card, but he/she still gets to "freeride" on the good credit from the account. Of course, the trust really has to be mutual, because the authorized user's credit rating could actually go DOWN if, for example, the main cardholder starts missing payments or revolving large balances.