Configure forms and rules in Microsoft Lists

Hi Gunter Danzeisen. The current Lists Rules behavior in the people picker is that all members in the tenant are searchable. If you attempt to add an email address of someone who is not in the tenant already, the person won't resolve and the rule can't be created. If you share the team site with an external user and they accept the invite, the user will then appear the people picker and thus can be added to a rule so long as they maintain access/permission granted via the invite. To be clear, the rule itself won't send or start an invitation. Hope that makes sense, Mark.