Mirror Group members are representatives from the Member States, designated by the highest level Member State authorities in charge of Research policy. Official representatives from Associated Countries can be admitted as observers. The chairman of the Mirror Group is designated by voting.

Members of the Mirror Group are responsible for validating the specifications report and the final set of recommendations to be considered as the policy-mix aiming to increase the involvement of private companies in the construction and the management of major European Research Infrastructures (existing and to be built). They will have access to working documents, and accept to keep in confidence all unvalidated working documents.