A company that makes home grocery deliveries is expanding to Houston this week, signaling growing confidence the concept will work in sprawling cities. [...] some of our least-dense cities have been the most successful. Houston will be the 14th city served by Instacart, which partners with grocery chains and relies on a cadre of part-time delivery workers dispatched by a computer program and paid on commission. The service is popular among parents with young children, busy professionals, senior citizens and office managers, he said. The company will likely have two full-time employees here to manage marketing, employee recruitment and customer service, O'Connor said.