This is definitely a case of “your mileage may vary,” but American Express is apparently offering Premier Rewards Gold cardholders and Platinum cardholders 10,000 Membership Rewards points per authorized user added, up to five. The offer code for this promotion is 5721.
You’ll want to call up Membership Rewards at 800-AXP-EARN, reference the promotion code above, and mention you heard that was a promotion to earn 10,000 Membership Rewards points for each new authorized user.
This offer should apply even to authorized users added in the past, as I was able to have them credit me the points for users added in the past.
Again, there’s no guarantee this will work, though it’s definitely worth a shot. It might take several phone calls and at the end of the day it might be targeted, so don’t get angry if it doesn’t work. But it is working for a lot of people on FlyerTalk, as reported in this thread. 50,000 points is easily worth $1,000, so well worth the effort.