Using the @user feature for notifying a user

There have been some discussion about this topic, but no, not supported.

You could try to pattern match entered text for existing users to notify them. But then you still have no auto-suggestion.

Unfortunately you won't be able to achieve a picker solution that appears inside another component such as a!textField() or a!paragraphField(), where users can type freely.  There have been some discussions on similar functionality prior, such as this thread, which involve having the user type out the entire @user.name, and then parsing the text to find matching accounts and log what you need to from there.  There can be validation included which will determine if the accounts typed are found or not, etc.