In this paper we present a case study about a community of practice's foundation and development among Italian teachers, researchers and university students who participated in a European project aimed at developing and testing innovative pedagogical models and technologies for collaborative knowledge building. Forty-five people (34 teachers, five researchers and six university students) participated in the community of adults that interacted for a school year both face to face and online. We analyzed interactions in order to study the roles, forms and distribution of participation in that community, and the content of teachers' reflections about the activity. The analysis focuses particularly on different modalities of participation between expert teachers (involved in the project from the beginning) and novices, novice and expert being treated as relevant dimensions according to Wenger's model. Conversations were transcribed and a qualitative analysis of face-to-face and online discussion performed. The diversity of roles and different modalities of participation between social factors involved in the community, in particular between novice and expert teachers, emerged from the analysis. In final focus groups, teachers underlined innovative potentialities as well as difficulties related to computer-supported collaborative learning, both in classroom activities and in teacher training. In these final focus groups, novice teachers participated in the community, becoming more competent and conscious partners in shared planning with the expert teachers.