Grid applications typically deal with huge amount of data and often the same data have to be transferred and processed on many resources. Nevertheless, the majority of existing middleware platforms for Grid computing do not provide suitable programming and communication models to make easy software development and to improve communication performances when a large set of receivers is involved. Some middlewares for wide area network computing, such as ProActive, provide the group abstraction to transparently deal with a number of similar receivers. We propose an extension of such a mechanism in order to improve its features for Grid environments. In particular, ProActive native groups have been extended both at programming and communication levels in order to support both different internal behaviors and high performance communication based on IP multicast. A case study shows the effectiveness of the new mechanism and its efficiency compared with the original one.