The "/subscribe" page:

This page is usually run by users who are not logged in. A logged-in user can add email notifications on the /alerts page. Access to this page by a logged in user (other than an administrator) results in a redirect to the /alerts page.

Administrators can visit this page in order to sign up other users.

The Alerts permission ("7") is required to access this page. To allow anonymous passers-by to sign up for email notification, set Email-Alerts on user "nobody" or "anonymous".