In general usage (and hopefully correct usage) a "notifier" is someone who notifies.
What is the word for the person who is notified?
For example: "Instruct each [X] to respond to the notification within 3 days."
While I expect that you're looking for something like notifiee, I'm not aware of any such word that would have that exact meaning. That being said, there are several good options that could work:
In the general case, (though these may overuse the notify root for your taste):
Instruct each of the notified to respond...
Instruct each notified individual to respond...
If the notification is a written message that they received.
Instruct each recipient to respond...
If the notification is some sort of pop-up in a program
Instruct each user to respond...
If those that are notified had to subscribe (e.g. to a mailing list) to receive the notifications, then you can call them subscribers.
I know this question is old and you've already accepted another answer, but for the sake of those like me who come after, the word you were looking for is notificant.
Notifyee (plural notifyees)
(rare, nonstandard) a person or organization that is being notified