Sustainability reporting is the process by which companies describe how they deal with their own economic, environmental, and social impacts, thus making stakeholders able to recognize the value of sustainable practices. As stressed in the Global Reporting Initiative guidelines, which act as a de facto standard for sustainability reporting, sustainable reports should take into account the stakeholders' view. In particular, engaging stakeholders is essential to carry out the materiality analysis, by which organizations can identify their own more relevant sustainability aspects. Yet, on the one hand, the existing guidelines do not provide specific indications on how to get stakeholders actually engaged; on the other hand, research on quantitative techniques to support stakeholder engagement in materiality analysis is scarce. Therefore, the purpose of this paper is the development of a quantitative structured approach based on multi-attribute group decision-making techniques to effectively and reliably support stakeholder engagement during materiality analysis in sustainability reporting. As it more strictly guides the reporting process, the proposed approach at the same time simplifies materiality analysis and makes it more reliable. Though any company can adopt the approach, small-and medium-sized enterprises (SMEs) are expected to particularly benefit from it, due to the quite limited implementation effort that is required. With this respect, the approach has been validated on a sample of Italian SMEs belonging to different sectors.