'''MCPO (Multicast/Multipeer Overlay)''' As an exemplary service on top of Ariba, we provide the MCPO service. MCPO's main purpose is enabling group communication mechanisms for applications using Ariba. This is accomplished by employing well-known Application-Layer Multicast mechanisms similar to the [http://portal.acm.org/citation.cfm?id=964725.633045&coll=GUIDE&dl=ACM&idx=J101&part=periodical&WantType=periodical&title=ACM%20SIGCOMM%20Computer%20Communication%20Review NICE] protocol. To be scalable even with higher numbers of participants, the approach uses hierarchical clustering, deviding the set of participants in layers of subsets. In its current state, MCPO finds its entry node by broadcasting a query. Therefore, it assumes a Base Overlay providing such a mechanism (as given in the One-Hop Overlay of Ariba). In the future, we plan to adopt DHT functionality as soon as it gets available in Ariba.