You have two choices to choose the messages to send to users.
1) Database Messaging,
2) SMTP (For which, the email id should be entered in the user's data)
to enable notifications, Click on the yellow color icon on the transisiton, which will allow you to choose the users.
You can find more info in the administration manual.
important the know is:
The notifications only are triggered when somebody else as the folder owner changes something.