Draft/published
By default, staff you're rostering to shifts will be informed of any additions and changes as soon as they happen.
In some instances, you might want to instead setup the roster as a 'draft', and then once you're happy, have any notifications get sent out to your staff letting them know of any changes.
To change a roster to 'draft', first go into the roster for an event. At the top-right of the page is an 'Options' button, where you can set the roster as draft.
Once set as 'draft', any changes made will not be sent out to your staff until you have marked the roster as 'Published', by clicking 'Mark as published' (see below).
Bulk changes
Changes to rosters, marking them as published or back to draft can be done via the events list - just select the events you'd like to change, and select 'Roster settings' at the bottom of the list in the 'choose an action' dropdown;
Hiding draft rosters
If you wish, you can set the configuration to hide rosters while they're in 'Draft', so they're only shown when the roster if finalised.
Your staff will still be shown events they're rostered to as part of their upcoming events, but the event will be flagged as 'draft' so they know the roster isn't yet finalised.
Do this this, goto the 'Configuration' page, and change the setting within the 'Roster settings' button at the bottom of the page.
More information about roster visibility is here