You can mention somebody's username in a comment with the @ symbol. We'll send that user a one-time notification that lets them know you mentioned them, and they'll be able to comment or follow the issue to receive future updates.
It's worth noting that mentioning people will only send them a one-time notification unless they choose to comment or follow the issue. Our philosophy is that people should not be able to auto-subscribe others to notifications. For more information, we have a blog post about notifying additional people.
If you type the @ symbol, Sifter will begin searching for matches for users first names, last names, and usernames.
Once you choose the user to notify, you'll see their name appear below the Update Issue button, confirming that they'll be notified when you submit the update.
Some people may have a notification turned on for when an issue they are following is referenced in another issue that they aren't following. If you reference an issue and someone following the referenced issue has enabled this notification, you'll see their name in the list of who will be notified.