This book introduces the idea of accidental collectives: the grouping of people that occurs as a by-product of the automated work of computers. Software has a growing influence in our lives automating and optimising mundane, time-consuming and repetitive tasks. In doing this, groups of people are automatically created as the result of classification and data analysis. Once grouped by the invisible agency of software, people interact and establish new relationships, generating new collectives and communities. With the support of case studies and real-life examples, this work explores the accidental nature of the generation of new social groups and questions the role of software in social interactions.