We discuss a wireless network providing service in a dense urban environment, where certain amount of users tend to move together in groups, e.g. in buses. We analyze a concept of mobile base stations (mobile picocells) mounted on vehicles carrying a large number of users, and their contribution to providing the service alongside the existing network with fixed base stations. The analytical and numerical results of blocking and forced termination probabilities are obtained and validated by simulations. The substantial improvement of network performance compared with a traditional service providing schemes is demonstrated.