American Express Membership Rewards seems to be on a points free-for-all lately. This Flyertalk thread details information on promotion code 5721, which offers 10,000 MR points for each additional user added to your existing Premier Rewards Gold personal and business cards, or the Platinum card. For the Gold card, you can add up to 5 additional users for free, and any additional users beyond that for $35 per card. Platinum cardholders can add up to 3 additional users for $175 total.
I tried calling the Membership Rewards line at 1-800-AXP-EARN this morning, and was shot down a total of 4 times before I was finally successful. Each of the first 4 customer service representatives I spoke to denied me on the basis that I wasn't targeted for the offer. On the 5th try, I spoke to a gentleman who asked me to confirm the promotion code 5721, which I did. He then transferred me over to the Customer Service Department, which handled the adding of the additional users.
At first, the lady I was transferred to asked for the Social Security number for each user, but I told her I didn't have them with me at the moment, and she said that was fine and proceeded to take down the names and date of births of each person. After about 5 minutes, she said they were approved and I would receive the cards within 5-7 days. I asked her to confirm that the promotion code was actually applied to my account, but she said she wasn't able to do that. I will probably wait a few days for the madness to end before calling MR back to confirm (seems like AMEX is getting flooded with calls regarding this promotion).
YMMV highly on this one, but never hurts to try! Just do so quickly before it gets shut down like the 100,000 Ultimate Rewards points bump on the Chase Sapphire Preferred card!
(EDIT: It looks like this promotion was originally targeted for American Express Platinum members in Argentina only. However, based on anecdotal evidence, it sounds like AMEX might be honoring the offer code for everyone through today 8/10/2011 only. If you get shot down by a CSR, don't bother arguing, just hang up politely and call back again until you get someone who is willing to help you apply the code.)