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Your staff/volunteers can do this by clicking the 'Can't attend anymore?' link in the roster information when viewing the event details. From there, a confirmation box appears to confirm their intention to withdraw from the event, as well as provide a reason or comment if available.
Alternatively, if you're tentatively rostered for an event, you'll see the following;
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Event specific - This facility can be turned on/off for each event by going to the 'edit event' page, and changing the 'Allow people to withdraw themselves' setting within the 'Staffing' tab of the 'Options' section. Changing things will override whatever has been setup in the group settings.
Default settings
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The default settings can be found in the group 'configuration' page, within the 'Roster settings' area, where you can change the setting as required within the 'Events' tabenable/disable the default settings for withdrawing yourself from the event, as well as how far away from the event this can be done - eg. 2 days beforehand / 6 hours beforehand / 1 week prior / etc.