Just tell your users to go to:
Options > Subscribe via email
That will take them to a screen where they can subscribe to emails and choose between the first post in new topics only or every message and whether to receive a daily digest or every message as it is posted.
You cannot force people to become subscribers, but once they opt to subscribe they appear on a list at:
Options > Users > Manage subscribers
and on that screen you can cancel their subscription.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.