Traditionally, wireless sensor networks were based on the assumption that all sensor nodes are participating in a single global application. However, it is desirable to use sensor networks for multiple concurrent applications. A key factor is the ability to dynamically structure the network into groups of nodes according to certain criteria, to later interact with the group as a whole. In this paper we present Scopes, a modular framework to support concurrent sensor network applications. A scope is a programming abstraction to declaratively define groups, which once established allow data exchange among scope members. The main challenges in attaining these goals are network dynamics such as variable node density, topology changes and node churn. We demonstrate the feasibility of the concept by achieving rapid scope propagation, high scope reliability and efficient data exchange.