Answer:
Go to the Sign in page, which you can access by selecting "Sign In / Register" in the top right corner of our homepage.
Select "Create Account," then enter your e-mail address, a password, and — if you're a member — the number found on your membership card. If you happen to have a Costco Anywhere Visa® Card by Citi, just look for your membership number on the back of the card, directly to the right of your picture.
Once you select the "Create Account" button, your account will be created.
Please keep in mind that being an active Costco member does not automatically register you on Costco.com. Your membership number is not linked to your account until you add it, either when you register or you update your account details under "My Account."