That sounds like a terrible scam and fraud.
Yep, scam and fraud. Scary to put your credit on the line for people that the bank won't give credit to.
The idea is for them to pay you and you add them as an authorized user so that their credit score gets a boost, but they (the people getting the boost) never have access to your credit card, the credit card #, or your information. They're not actually using your lines of credit.
Still seems to be a risk to me, and if found out by the banks your lines would probably be closed.
Edit: Shareyourcredit has an A+ rating with the BBB, and scamadviser.com states it seems to be safe to use. But again, still risky IMO.