The increasing number of man-made crises presents relief organizations with serious challenges. For one, paid relief workers are increasingly scarce, making disaster volunteers essential components of official endeavors. But there are several issues concerning communication with as well as coordination and organization of disaster volunteers that hamper their deployment. We propose a design theory for mobile crisis response systems to support the management of disaster volunteers, with a focus on the current refugee situation in Germany. We derive requirements from interviews with relief organization employees working at the national level and those operating in the field, as well as with disaster volunteers. We translate requirements into both functional and non-functional design principles and map them to adequate design features, which we implement in a mobile crisis response system. As a practical contribution of our research, we provide specifications that define the form and function of a corresponding system.