Organizations increasingly rely on new forms of organizing for innovation that are semiformal. A typical example is represented by internal web-enabled ideation platforms through which employees can provide innovative ideas, interact and contribute to the innovation process. However, limited attention has been placed on investigating the organizing structures of these initiatives. Based on data collected from an online ideation platform of a large European organization, we combined cluster analysis, social network analysis and interpretative content analysis to identify which roles employees assume and how they differ in the content and outcomes of their contributions. We identify a typology of five employee roles characterized by different behavioral contribution patterns and which differ in their communicative practices. Moreover, we identify a key difference in the contributions made by central and marginal members. While more active employees play a key role as innovators, being more successful in their ideation efforts, the key contribution of more marginal members lies in the elaboration phase of the idea journey by providing different types of feedback directed at supporting, refining and improving submitted ideas. Implications for innovation management research and practice are discussed.