As a Costco member, you can bring one guest with you when you shop at the warehouse. However, only you can purchase things from Costco using the membership card. If you have a valid Gold Star membership, you can assign a membership to one person in your household.
To add one more person, you need to upgrade to the Business-level membership. It comes with an additional fee and you must pay for it at the time of your upgrade. You can also get a guest pass, which is valid for three months.