Spontaneous volunteers provide essential assistance during crises yet integrating them into official response structures remains challenging. This paper examines how a mobile application can support spontaneous volunteers’ recruitment and coordination in storm surge scenarios. Following the design science research paradigm, we conducted a systematic literature review and six semi-structured interviews with citizens and emergency stakeholders to identify key issues. From this analysis, we derived design principles, which were instantiated in a high-fidelity Figma prototype. The prototype was evaluated with twelve participants using scenario-based walkthroughs with concurrent think-aloud and qualitative interviews. Findings confirm the value of centralized information, flexible registration, and accessible safety preparation, while also highlighting needs for moderated communication, configurable recognition mechanisms, and features for visual salience and social engagement. We articulate seven design principles that balance autonomy and oversight, reduce cognitive load, and foster trust. These contribute actionable guidance for designing digital systems that empower citizens and enhance disaster resilience.