The increasing number of Web Services provided by several service providers demands for effective, scalable and accurate mechanisms to search and select the most appropriate service, fulfilling some user requirements.Distributed, semantics-enabled infrastructures for service discovery could satisfy these requirements, even if the promise of dynamic selection and automated integration of web services is far from being kept.In this paper we present a P2P overlay infrastructure, aiming to better support a scalable service discovery process.Overlay groups are formed by clustering together peers offering services which are semantically related: a search request for a service (query) will be handled by the group where it has the highest chance to be satisfied.The main focus of the paper is on the groups creation and maintenance strategy, and on the query routing mechanism. A system prototype has been implemented and validated through a case study.