We propose a decoupled distribution paradigm for connecting large numbers of content publishers and consumers called Implicit Group Messaging (IGM). Unlike traditional multicast or publish/subscribe messaging, IGM delivers content to "implicit groups" of consumers matching combinations of attributes specified by the publisher at the time of publication. The contributions of this paper include a characterisation of IGM encompassing the types of groups that must be supported, and an innovative distributed P2P implementation that we compare via simulation to a functionally equivalent client/server approach. Our results show the P2P model to efficiently support a range of implicit groups with acceptable delivery times and vastly less maximum peer and link stress than a client/server approach.