I'm a little confused!

Holger_Duempelmann , apologies for the confusion. The "first opinion" is the most accurate.

The public Mesh documentation covers the current implementation and limitations here as follows: 
"
...Because all users who will be participating in immersive experiences together must have accounts together in the same Microsoft Entra (formerly Azure Active Directory), you may want to consider using a separate tenant if you want to collaborate with people outside your production tenant; guest accounts in Entra do not have access to Mesh experiences. There is no technical barrier to creating user accounts in a production tenant for people who do not work for that organization, but there may be strong business reasons against doing so. However, creating additional tenants will increase complexity for admins and users to manage accounts, may also incur additional expenses for licensing and domain management, and may require additional process within your organization.
"

Feedback on the need for "guest access" (I use one term to cover the myriad use cases, please don't assume this choice of words implies future implementation details) has been heard, and the Mesh team is reviewing if/how that can be achieved in future.