Generally, online support groups are viewed as low-threshold services. We challenge this assumption with an investigation, based on Conversation Analysis and Membership Categorization Analysis, of contributions to an online support group on eating disorders. In this analysis we show how a new member interacts with existing members in order to display legitimacy for membership of the group. The group operates as a Community of Practice, since membership is organized as joined participation in a writing practice. It becomes clear that becoming a member involves subscribing to normative requirements, centrally, displaying the insight that you are ill. In the case we focus on, this involves the requirement to leave pro-anorexia as a membership category behind. The novice does not yet seem ready to subscribe to this norm and thus the threshold for seeking support is heightened. © The Author(s) 2010.