Knowledge sharing within an organization allows it to develop a competitive advantage over its rivals. However, sustaining and motivating members of knowledge sharing groups to contribute knowledge is a challenge for many organizations. To address this problem, organizations have implemented new knowledge sharing tools such as Wikis to facilitate knowledge contribution, although these have had varying success possibly due to the importance of social identity in knowledge sharing behavior. This study examines how similarity in work roles and work familiarity between members affect various dimensions of social identity they individually perceive, and in turn, affect knowledge contribution. Field data gathered from a knowledge intensive organization provide evidence that similarity in work roles and work familiarity have a strong influence on social identity, which in turn affects knowledge contribution.