Just enable it in Dashboard > Comments > Settings admin page and use.
I enabled it, but couldn’t understand how does it work. Can I tag members? I tried with their names, usernames but nothing happened.
Quick tags are designed for HTML content (adding link, formatting text…), this is not for User Mentioning.
More Info: https://codex.wordpress.org/Quicktags_API
wpDiscuz has an addon for this: wpDiscuz User & Comment Mentioning
