Service oriented applications feature interactions among several participants over the network. Mechanisms such as correlation sets and two-party sessions have been proposed in the literature to separate messages sent to different instances of the same service. This paper presents a process calculus featuring dynamically evolving multiparty sessions to model interactions that spread over several participants. The calculus also provides primitives for service definition/invocation and for structured communication in order to highlight the interactions among the different concepts. Several examples from the SOC area show the suitability of our approach.