I just recently added my partner as authorized user to my three credit cards (the oldest of which is uhhh... maybe 5 years old, I'm not sure, and the newest is less than a year old) bringing his total credit line from $500 to $27000.
I have an 800+ credit score (according to FICO as reported by my credit card issuer) and never carry a balance. His mid-600's score rose by 40 points (according to Credit Karma) within days of my adding him.
One of my three cards asked for the last 4 of his SSN, the others just asked for his name, address, and birthdate. All three cards are "major" credit cards. Two of the card issuers mailed a card in his name to me, and one mailed it to him (we live together, but that was how the mail was addressed).
My credit score has and will remain unaffected (assuming he doesn't charge up a balance on my cards, which he won't).
My partner has no legal liability for any charges he makes on my cards, that liability remains mine (so don't do this for anyone you don't trust with your credit card), but the credit line and account length do show up on his credit reports and are factored into his score. Likewise, if there was heavy utilization of the cards, that would affect his credit score (even though he wouldn't be liable to PAY the charges). Also, I were to make late payments on the cards for which he is an authorized user, that would hurt his credit score as well as my own. But he could at any time call the card issuer and have his name removed as an authorized user, which would make the card disappear from his credit history. So basically, if I were to fuck up royally and start charging up high balances and making late payments, he could get that off his credit history immediately and go right back to where he was before I added him.
I could remove him at any time as well. However, any charges he were to make during the time he was an authorized user would still be my responsibility to pay.
As I understand it, a good way to build credit is to take advantage of the bump in credit score as an authorized user on someone's high-limit, low-balance, long standing credit card, and then apply for a better (higher limit) credit card on your own that you might not have qualified for before. Or apply for a credit line increase on cards you already have. That way, you'll be building credit history on your own card.