BuyWithMe uses social networking to aggregate large groups of buyers to make purchases in bulk — thereby lowering prices at major retailers. It has just raised $5.5 million in first-round capital to scale its user base and forge more vendor partnerships. The service also sends out email alerts to users calling their attention to sales and other deals in their local areas (right now it only operates in Boston, Washington, D.C., and San Diego). Based in Boston, the company received the recent financing from Matrix Partners. It was previously backed by family and friends.