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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;
In either instance, clicking the 'Cant attend anymore' link will show the following box for your staff & volunteers to then use to withdraw themselves from the roster.
When submitted, the system will mark their name as 'Withdrawn' from the event, and any reason/comment given will be added to the roster notes for the person.
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