You will get 1,000 bonus starpoints for adding 1 additional cardmember, and 2,000 if you add 2. In my experience (admittedly contrary to the fine print quoted below) I have not had to actually activate and use the additional cards in order to get the bonus points, but your mileage may vary.
With the new Fico ’08 algorithm, being an additional cardholder supposedly no longer benefits your credit score in any way. It will show up on your credit report, but it is no longer calculated in the score.
Be sure to print out a copy of this offer just in case AMEX does not credit you the bonus points automatically…
“1,000 bonus Starpoints will be awarded to your Starwood Preferred Guest Account 8-12 weeks after the first purchase on an approved Additional Starwood Preferred Guest Credit Card submitted with this application. The Additional Cardmember must make their first purchase with the Starwood Preferred Guest Credit Card within one year of Card approval. Bonus offer applies to first 2 Additional Card added on this application only.”