This paper addresses the challenge of providing scalable end-to-end differentiated service for group communications. While bandwidth broker (BB) has been proposed for guaranteed services in unicasting, no work has been done to extend it for multicasting. We propose a single-layer BB architecture for intra-domain multicast, and a multi-layer BB for end-to-end services across multiple network domains. Databases in the BB, the join and leave procedures, and their associated control messages are carefully designed. Complexities of the two proposed architectures are formally analyzed. It is shown that, comparing with the single-layer BB architecture, the multi-layer architecture results in much lower join costs and control-message complexity, and is thus highly scalable. Performance is evaluated via simulation of a multi-domain network with heterogeneous receivers. The proposed multi-layer BB architecture successfully provides end-to-end service differentiation in terms of throughput, end-to-end delay, and packet losses. We believe that this work is significant towards offering end-to-end differentiated-service group communications in large-scaled networks.