MasterCard World Elite card members are eligible for a free 1-year membership to National rental car's Emerald Executive Club
However, it looks like anyone can get the offer just by following this link.
Expires: Feb 28, 2017
Emerald Club Executive® status is yours with 12 paid rentals or 40 paid rental days in a calendar year.
You'll enjoy all the benefits of Emerald Club® membership plus a higher level of privileges, including:
Choice of Fullsize or better cars from our exclusive Executive area.
Guaranteed upgrades: You always pay for one car class less than the car you reserved.
One Free Rental Day with six rental credits.
: for those who perhaps don't know...
rentals at Enterprise also qualify toward the free rental day reward, and the 12 times/40 days requirement to retain executive level after feb 16 expiration.
just make sure they add the Emerald ID during reservation or by calling National after rental is complete.
I already have National Executive membership, so I can't give complete details of the signup steps
but it doesn't look like they verify at any point if you are indeed a mc elite cardholder
good luck, and have fun y'all!