University of Oulu University of Oulu University of Oulu University of Oulu This document introduces a mechanism for managing peer groups in structured peer-to-peer (P2P) overlay networks. Group management enables efficient and secure interaction between the group members (peers), by controlling the scope of propagation for the group- specific P2P messaging and other communications. The mechanism for group management, introduced in this document, is based on a multiple-overlay scheme. Each group is a separate overlay network with its own address space, resources, and message routing information. A common overlay is used for sharing information about the group overlays (sub-overlays).