Access Group in power Bi Service

Hi Paolo_lito 

For the external user you have to give them your tenantID for them to log in with, then they will be able to see.Click on the ? and then About Power BI

And then click on the Tenant URL

Hi, Paolo_lito 

To help allowed guests sign in to Power BI, provide them with the Tenant URL.

I hope this explanation can help understanding
External users first have their own tenant, such as AA.onmicrosoft.com. When you invite external users to your tenant BB.onmicrosoft.com, maybe users will be invited to other tenants CC.onmicrosoft.com, then how to decide on Which tenant does the user enter when logging in?

Therefore, the user will only enter his tenant when logging in. If you want the user to log in and enter your tenant, then you need to provide a URL. After the user logs in through the URL you provide, he correctly enters the tenant you want him to enter instead of his own tenant.

refer:

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Best Regards,
Community Support Team _ Zeon Zheng

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