You just need to include their username in the issue.
When you @mention a GitHub username anywhere in the context of an issue or pull request, that person is notified and subscribed to future updates.
So if you wanted a GitHub user by the username, tornadosandwich, to be notified that you want them to see the issue, just put @tornadosandwich in the body of the issue and it will notify them.
Would like to know what @tornadosandwich thinks about this.
This will only work if the user has allowed it. If they never respond or acknowledge being included they may have unchecked both boxes in their account settings under the Notification Center:
Participating
When you participate in a discussion or someone brings you in with an @mention.