The Rules For Adding People To Your Costco Membership - Tasting Table
Briefly

Costco's membership policy allows one additional person per regular account, provided that individual is 16+ and lives in the same household, ensuring exclusivity.
Business Membership accounts at Costco can add up to six additional cardholders at $65 each, allowing greater sharing potential than regular memberships.
Costco has tightened regulations on membership card sharing and is now stricter about checking membership at entrances and self-checkouts.
Annual membership for personal accounts at Costco starts at $65, a price that was recently increased—the first hike in years.
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