It seems that Confluence has the same design bug as JIRA when it comes to sending notifications to users that never logged in.

This bug is very annoying when you deploy the product to a large enterprise, when only few people are users from the beginning and slowly they start to add watchers, mentions, emails to people that never logged to the system.

Obviously these people would become users if they would ever login, but they will never be notified... so it is a chicken and the egg problem.

I see only two workarounds on this: prevent the usage of non-users in all dropdowns or even better, auto creation of their accounts (default groups assignment) when they are used (by anyone).