Disaster management research increasingly focuses on how to collaborate with emergent volunteers in order to support formal disaster agents in the nonprofit sector (Whittaker et al. in Int J Disaster Risk Reduct 13:358-368, 2015; Strandh and Eklund in J Conting Crisis Manag 26(3):1-9, 2017). In a disaster context, we regard emergent collaboration between these agents as an appropriate approach for structured self-organization (Simsa et al. in Nonprofit Volunt Sector Q, 2018. https://doi.org/ 10.1177/0899764018785472) and hence for providing sustainable disaster relief. Our research seeks to identify which factors facilitate such emerging collaborative efforts. Using survey data from Austrian refugee migration in 2015/2016, we examine how social capital components affect the collaborative efforts between nonprofit organizations (NPOs) and emerging volunteers on a team level. Data evaluation is based on regression analysis. We provide empirical evidence that social capital components like 'avoidance of misunderstanding' and 'interaction frequency' enhance the collaborative efforts between NPOs and emergent volunteers. Furthermore, the study highlights the roles of 'emotional intensity' and 'intimacy' in collaborative disaster relief performance.