In addition to their own membership card, Primary members (and Affiliate members on Business memberships) receive one free Household Card they can assign to someone over the age of 18 who lives at the same address. Household Cardholders will be asked to present proof that they live at the same address as either the Primary or Affiliate Member when picking up their card at the membership counter.
To add someone as your Household Cardholder, visit the membership counter of your local Costco. Or, if your Costco.com account is Verified, you can manage your Household Card on the Account Details page of My Account.