The "/subscribe" page:

Allow users to subscribe to email notifications.

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".