I don't think there is such a code for any of the rental agencies that are in Sam's travel program.
All current Sam's Club discount codes are on a page of the Sam's Club Travel Guide, which is a quarterly publication that you pick up from the racks of member info near the exit of each club location. I just checked the current issue (May-July Summer 2005) and there is no such code on the rental car page.
What you may have seen was someone renting with Sam's Club dicscount codes but getting the extra driver fee waived for another reason, such as being a member of Thrifty's Blue Chip Club or Dollar's Fastlane Club. Membership in many of these frequent renter clubs gets the extra driver fee waived for one extra driver.
One or two of the major rental companies (can't remember which ones right now) don't even charge an extra driver fee for the renter's spouse, so you might have heard that as well and thought it was because of a Sam's Club code.