http://www.nwa.com/offers/referafriend/
Rule #1 states in part:
"Sponsor must be a WorldPerks member whose mailing address is in the United States. Referred Friend (Friend) must be a WorldPerks member who has not earned WP miles for travel since January 1, 2004 and whose WorldPerks mailing address is in the United States."
If you don't have a WorldPerks number yet, you can't participate, so that's why I got all of our accounts set up today.