Sharing Gemini Notebooks with domains that are part of your organisation

I would like to be able to share Gemini Notebooks (NotebookLM) with users that are within my Google Workspace organisation, but use a completely different UPN.

For example, in my Google Workspace organisation, I have two domains registered in the “Domains” section. I have the primary domain: “company1.co.uk” and an additional domain: “company2.co.uk” (this is all still within the same Google Workspace organisation).

Let’s say I (me@company1.co.uk) create a Gemini Notebook, with a source that is on a shared Google Drive (a Google Doc). I have a requirement to share this Gemini Notebook with another user in our Google Workspace environment (you@company2.co.uk) (you@company2.co.uk has view access to the Google Drive area where the Google Doc is by default because of where it is located).

If I share this with you@company2.co.uk, the sources are not read. It states “Source:0” in the bottom right.

I understand that when Google states that Gemini Notebooks cannot be shared out of the organisation, should that really read “cannot be shared with another domain name”?

The new Gemini sharing options do not help either. If I add “company2.co.uk” as a Trusted Domain in the “Allowlist”, I am advised that you “Can’t add your own domain or domain alias as an allowlisted domain.”

I do not want to enable public sharing.

How can I best enable sharing with additional internal domains for our users?

I am aware of the workaround with Google Groups, but I have not tested this yet. It is not solution I want to adopt.