P2 is a WordPress theme from Automattic that provides a great format for threaded group conversations on the front end of a WordPress installation.
Mentions is one of the cool features of P2, allowing you to add other users to your status update to bring them into the conversation or to just catch their eye.
This plugin hooks in to the assignment of these mentions and works with wp_mail() to email users when they are mentioned. Filters for email subject and body are included so you can customize the message to your liking.
This plugin is not available on the WordPress.org plugin repository, but can be installed by uploading the zip file provided by GitHub for this repository to your WordPress installation. WordPress will extract the files as needed and you will then be able to activate the plugin.