Some encouragement for those who don't get it on the first call:
I had to call MR three times - All in the space of half hour.
First call: As soon as the rep heard me talk about the 10K for additional cardholders, she told me that it is only available for targeted accounts.
So I hung up and immediately called again.
Second call: The rep had no idea what I was talking about. Spend 5 minutes looking up offer code 5721, and came back saying that it must be something cardmember services deals with, so he transferred me to that department. The rep from that department had no idea what I was talking about, so looked it up for 5 minutes and came back aplogising that she couldn't help me.
So I hung up and immediately called again.
Third call: As soon as the rep heard me talk about the 10K for additional cardholders, she says "sure, I can help you with that!" She didn't even ask me for the offer code 5721. She notated my account, and she then transferred me to cardmember services to add the additionals. When the cardmember services rep heard that I want to add 5 additionals, she says, "this must be for the 10K offer for additional cardholders, right?" She was more than happy to help, and I did not need to supply SSN's.