Signed in as the Administrator, click on the Reports tab and click on the Participants Report.  This will give you a list of all the volunteers for the events selected, as well as their email addresses.  You can then select those emails, copy and paste the addresses into the To field in your email program (or into the Bcc field, if you prefer to keep the recipients from seeing each others’ email addresses).  Beware that if volunteers signed up for more than one shift, you may want to check and eliminate duplicate email addresses.  You can do this manually in your email program, or you can use Excel’s Sort and Filter functions before you copy the addresses into Outlook.