Share workspace outside organization

Which steps do I have to follow if I'd like to add an external user to one of my workspaces to visualize or maybe to collaborate in editing a list of reports?

I checked I have admin portal permissions to share externally, but when I edit the email of external user with his PRO account it warns me "the email is not valid or duplicate".

Do I then have also to share the reports inside that workspace or the permissions over reports, datasets and dashboards are inherited?

Do I miss something? I can't fin a step by step best practice in the documentation.

Thanks