If the Capability Approach is to be a suitable framework for assessing sustainability, as is currently being discussed in Ecological Economics, it is crucial to grasp how the individual and collective levels are linked. The aim of this article is to investigate how individuals interact to create a collective actor. Thus, the article analyzes the process of setting up a grassroots organization that aims to make an active contribution to sustainability implementation in a given locality. The article specifically examines the process involved in the development of collective agency and collective capabilities and identifies the tensions between the individual and collective levels. The empirical analysis along with the theoretical discussion enables us to put the concepts of collective capability and agency in the context of Ecological Economics. Finally, this article opens up new areas of research to further our understanding of how the individual and collective levels can be linked with a view to sustainability implementation at the local scale.