Sharepoint Online external Users dont show in People Field

Hi everyone, 

we have a Sharepoint site where external sharing is disabled. In this site is a list with a people field in which we would like to be able to select external users. 
However, only internal users are displayed, not guests. I also tried to specify the email address of the external users, but then I get the error that external users do not have access to this page.

The external users are all invited to our Azure Active directory and have accepted the invitation. They are also already members of Teams etc.

How can i add external users in a People Field inside this List?

Thank you