Can I bring guest and can they pay for the items they purchase

I haven't seen them restricting the amount of people, and they can pay under card but I would use the self checkout so they don't give you any hassle unless you use cash. This is because the card holder name for the membership is suppose to match the name of the card used.

I believe you can bring 1 guest and they can pay as long as you use your membership card. You can also get them a CostCo cash card and they (2 people) can come without you.