If you have a child (over the age of 16), parent, sibling, spouse, etc., who would like the privileges of shopping at Costco, as long as they live at the same address as you (and can prove it with an ID or driver's license, or a bill with their name on it — they might be asked), you can add them as your free household card holder, and you can do it online. Go to the Account Details section after signing into Costco.com, follow the prompts, and then head to the store to pick up the card. Or, you can have an employee add them at the membership counter in-store.
Can Costco Membership Be Shared?

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Have a friend with a Costco membership and a really big car trunk? You can have them take you with them to shop at the bulk store, as one of two guests that are allowed to accompany each member. There is one caveat, though — as a guest, you can't actually make any purchases. Only members can buy things in Costco stores, so you should both be prepared for the cardholder to pay for the entire cart; you can then make arrangements to pay them back.
Now, some customers have reported a method of sharing their memberships that isn't exactly ethical — basically, if there's someone you want to add to your account as the household member, but they live at a different address, you can go into your account and change your address to theirs, then sign them up. Then, when they go to pick up their card, the address matches their ID. Would it work? Probably. Is it street legal? Not exactly. Use this information at your own peril.
But there is actually a way to shop at Costco without a membership at all; you can use the Costco storefront on Instacart. There's an option to enter your member ID, but you don't have to. Just know you'll pay higher prices than in-store, to offset the fact that you haven't paid for that Gold Star or Executive membership.