You can enable or disable the notifications that each user level will receive on the Event and Project level.
From the Project level, navigate to the Settings tab. Once there, click on the Notifications tab in the left sidebar, and you can edit the notifications that will be received by checking or unchecking the box. 
For Event level notifications, go to the "Settings" tab of the event you want to set notifications for. In the left sidebar, click on the "Notifications" tab. Here you can change the event level notifications received for Moderators, Observers, and Participants.
One important thing to remember is each user must still opt into receiving notifications. If a users opts out of receiving any notifications, this setting will not change their individual preference.