This paper reports on an ongoing international research about MOOCs for in-service mathematics teacher training. We describe and analyse two different experiences of this kind: the Italian MOOC Geometria and the French MOOC Efan Maths. Both MOOCs aimed at supporting the teachers' professional development through a suitable mediation of technology and at triggering as much as possible the teachers' engagement so that they could develop from a non-community towards one or more communities of practice. As authors of this paper, we are members of the trainers' team of their respective MOOC and we also participated in its design. Starting from our methodological choices, we want to propose some reflections about design principles of MOOCs for mathematics teacher training in order to foster participation and collaboration among trainees and to efficiently assess this kind of engagement.