The discussion document Teachers of mathematics working and learning in collaborative groups (IPC, 2019) identifies "Roles, identities and interactions of various participants in mathematics teacher education" as one of four major themes needing further elaboration. In regard to this Theme C, the document indicates, among others, that collaborative groups can include different 'actors' in various combinations. These actors can have a variety of roles, which can shift over time. In collaborative interactions, the learning of all participants is also important. The document indicates six 'actors' in an exemplary way.It is easy to increase this list by including, besides mathematics teachers themselves, other individuals such as teachers of other subjects, lead teachers or teacher leaders, department heads, principals, parents, teacher students, students, critical friends, facilitators, coaches, mentors, mediators, designers, multipliers, mathematics teacher educators, mathematicians, researchers, administrators, superintendents or policy makers-and even this extensive list is not exhaustive at all. In addition, also organisational entities like departments, schools, school boards, districts, committees, ministries or enterprises can be environments relevant to a collaboration: for example, by influencing the goals, processes and results of collaborative activities which take place in projects, programs, teams, communities of practice, networks,