Youd probably have to print out a confirmation email of some sort from Groupon or get in touch with them so that they can reissue you a card. If Sams cant find you in the system, then contacting Groupon is your best bet.
What Dennis said, under your Groupon account, your purchases from Groupon should still be listed. Although, Sam's should have you in the system if you have their membership card.
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