All logged in users are able to mention others users, This functionality is implemented for:
Posts (In groups, on profiles or in the community)
Comments (on posts, topics, events)
Start typing an @ followed by the name, username or nickname (when this field is enabled) of the person you want to mention; a list of (max 100) results will appear. It’s also possible to narrow down your search by providing a combination of the first and last name of the user you are searching for.
The user who has been mentioned, will receive an e-mail and personal notification.
When being mentioned in a comment, you will also receive notifications of replies to that comment.
Mentions are also used to reply on a reply.
Manage e-mail notifications settings in your account!