Hey Shaun,
Only designated brand managers in an enterprise can publsih official kits. For that, the office app admin for your enterprise needs to set up the enterprise brand manager policy (via config.office.com) and add the security group containing the brand managers for the organization. Here's a step by step guide for the policy setup process.
Once the policy is configured, the brand managers in the security group provided by the admin, will see a publish option in the brand kits they created. Clicking on this publish button will help you share this kit with all Copilot Create users in your organization and use those in branded artefact creation. Please note editing of these official kits can only be done by the designated brand managers, other users will only be able to access these kits and their assets.