The offer can be expanded to a total of 25,000 miles by adding 3 cards for others.
From T&C "Additional bonus miles when you add up to 3 cards for free for others on your joint checking account, with each new cardholder getting 2,500 bonus miles after their first qualifying purchase"
From Gary Leff's Blog
http://blogs.flyertalk.com//blogs/vi...s/continental/