This proposal is developed out of the concern for the need to measure and improve the effectiveness and maturity of the governance of organizations where they operate as groups. This document will provide a maturity model for the governance of groups of organizations based on ISO 37000:2021 — Governance of organizations — Guidance and ISO37004:2023 — Governance of organizations — Group Governance maturity model — Guidance. This document will provide guidance on measuring and evaluating the governance maturity of a group of organizations, against the ISO 37004 Governance Maturity Model. This document would apply to all types and sizes of organizations which currently operate in, or are considering operating in, a group of organizations (group). This document would also apply to a group of organizations which can include one or more organization(s) which is itself, part of a group of organizations. For the purpose of this proposal, a group of organizations is an arrangement of two or more organizations in which one organization is a member stakeholder (3.3.2, ISO37000:2021) of another. Where an organization is a member stakeholder of another (for the purposes of clarity in this document, the organization is termed the “holding organization”), it has a legal obligation or defined right to make decisions in relation to the governing body of the other organization (for the purposes of clarity in this document, this other organization is termed the “ancillary organization”). In this way, the holding organization is able to impact the governance of the ancillary organization through its governing body. It is as a result of this potential governance impact that the governing body of the holding organization should be responsible for its governance impact, while at the same time, respect the authority, accountability and autonomy of the ancillary organization and its governing body. This proposal addresses the concern for the need to measure and improve the effectiveness and maturity of the governance organizations in a group, which includes the way in which governance is impacted as a result of their operating as a group.