Note: This isn't the actual type of app, it's just a similar situation.
If I have a mobile chat app in which a user creates an account, creates a new conversation and is allowed to add others via email. If the email does not belong to a user of the app, then an email will be sent to the person inviting him or her to sign up. If the email belongs to a registered chat-app user, then he or she is just included, and like other chat apps the message will appear in the message list of the app.
What should be the expected behavior in a scenario where the person who creates a new conversation and only adds emails that do not belong to any registered users?
Here are my thoughts so far. I do not want to require the user to add at least 1 other person who is a member. I also wanted to try to avoid having to have a list of "friends", and only allow them to start a conversation if they have 1 or more friends in their list. The reason is that I did not want to implement a friend request/invitation system. It's not a social network.