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This paper proposes a novel lookup service enabler for presence-based applications and its architecture in the 3GPP IP multimedia subsystem (IMS). The new service enabler makes possible the realization of a wide range of new (or enhanced) end-user applications. It allows end-users to discover groups of persons (or objects) that meet specific constraints (e.g. have specific interests or abilities, are available at a specific time at a specific location, and/or have specific affiliation(s)). The architecture relies on a novel abstract presence object (i.e. a non-user presentity) that represents a group of entities that may belong to different domains. The current presence model and presence information data format (PIDF) are extended to support this object. A new entity (i.e. a lookup server) is introduced to enable the lookup service through a seamless aggregation of the results collected from the different presence servers deployed across the implicated IMS networks. A proof-of-concept prototype was built and is described in the paper.