Focus groups are a mainstay of retail, but Walmart's warehouse club is taking it to another level in its battle against Costco. Sam's Club is assembling a large and growing community of shoppers to help it develop new products as it races to expand the value of a membership. The company said Thursday that its member engagement community has topped 150,000 participants, up from 50,000 two years ago, after it opened the previously invite-only program to all members in February. "As we enter into