The auto complete functionality will just work without any configuration. You can disable it by adding this to the $editorOptions as you set the editor up in your view:
PHP:
'extraClass' => 'NoAutoComplete',
As for the process of turning @tags to a username and alerting users, just look at the existing code:
XenForo_DataWriter_DiscussionMessage_Post::_preSave();
...is a good place to start