To start a conversation with non-members I have to create an email and add them with their email address. When the email is sent, they are included in the conversation and receive all replies.
As long as this topic remains within my inbox I can see that those newly created staged users are part of the conversation even if they have not yet replied.
But as soon as I make this a public topic this is no longer the case. But still they receive any reply to this topic although this is in no way visible. This is a bit dangerous because people replying may not be aware that some external users will see the post.