American Express is offering 10,000 Hilton points for Amex Hilton HHonors cardholders when they add an additional authorized user.
Link: Earn 10,000 Hilton Points for Adding Authorized User [YMMV]
The offer is targeted and sent out via email, so be sure to check your inbox if you currently have the card.
Per the terms the offer is targeted, but many people have been able to log in via this link and see the offer on the left side of the page. The bottom line is your mileage may vary, but it's probably worth a shot for 10k points.
If you potentially qualify you will see details for the offer on the left side of the page where you can enter information for an authorized user.
The offer states that it will will expire on 7/20/2016, so make sure you complete it before then. You should received the 10,000 Hilton points 6 - 8 weeks after you add an additional card.