Confluence has an interesting @mention feature: by typing @ before a person’s name, the system autocompletes the name for you, makes it a link to the user’s profile and sends him/her a e-mail with a link to the page s/he has been mentioned on.
However, if the user has never logged into Confluence, this feature won’t work. To force Confluence @mention a user, login to Confluence Admin panel, goto Users, find and click on the user, click on View Profile and click Administer User.
Now you may @mention this user on a Confluence page. You have to repeat the process for each user you want to appear in @mention suggestions.