Many clustering protocols for mobile ad hoc networks (MANETs) have been proposed in the literature. With only one exception so far [1], all these protocols are proactive, thus wasting bandwidth when their function is not currently needed. To reduce the signalling traffic load, reactive clustering may be employed. We have developed a clustering protocol named 'On-Demand Group Mobility-Based Clustering' (ODGMBC) [2, 3] which is reactive. Its goal is to build clusters as a basis for address autoconfiguration and hierarchical routing. In contrast to the protocol described in [1], the design process especially addresses the notions of group mobility and of multi-hop clusters in a MANET. As a result, ODGMBC maps varying physical node groups onto logical clusters. In this paper, ODGMBC is described. It was implemented for the ad hoc network simulator GloMoSim and evaluated using several performance indicators. Simulation results are promising and show that ODGMBC leads to stable clusters. This stability is advantageous for autoconfiguration and routing mechansims to be employed in conjunction with the clustering algorithm.