The most consistently misunderstood aspect of Costco membership is not the annual fee, not the Executive upgrade math, and not the coupon book redemption process. It is the membership sharing rules — specifically, who can use a Costco membership, how, under what conditions, and what happens when the rules are violated. The confusion is understandable. Costco membership is a household institution, and households are complicated. Spouses, adult children who move back home, coll