REUTERS: Wal-Mart Stores Inc is expanding free grocery pickup service to several markets in the U.S. as it seeks to capitalize on its network of physical stores amid growing competition with Amazon.com and others investing in home delivery. The world's largest retailer said on Tuesday that it would start offering curbside pickup for groceries ordered online in eight new markets this month - including Atlanta and Salt Lake City - with more to be added in the coming weeks.