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!
The amount of results showing up when starting to type, can be controlled via a setting. This is use full for platforms with a large user base, where it is harder to find specific users.
The default 8 might return results without the person your are looking for, for example for searching for 'Ana' if you don't know her last name (or is she hasn't filled it in).
if you run into these troubles, increasing the numbers of results might help!
This setting can be found here: /admin/config/people/mentions