For a while now, you have been able to @mention a user when discussing issues, pull requests, and commits.

Now you can use the same convention to mention a Team from an organization! Just enter the @organization/team-name using the fancy autocompleter, and all members of that team will get subscribed to the issue.

Note that once a member of a team is subscribed to an issue or pull request in this manner, they can manage it in the same way they can manage individual @mentions. In case you forgot, you can tweak notifications in your Notification Center under Account Settings.