To customise roster-notifications (eg. when someone is added/removed to an event, or their roster is updated), 'Configuration' page (within the 'Settings' menu), click the then in the 'Events' tab, there's a button called 'Notifications' button ( at the bottom of the page) within the 'Events' tab within the Site Configuration page. This will open up an area which lets you change notification settings, as well as customise (or use the default message) the email messages as needed.
You can customise the notifications that get sent to people when their rostered status for an event changes.
To do this, select the roster status that you wish to customise the notification for, and then enter the message that you want to be sent.
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