To send an email out to all volunteers who are in one particular job, you can use the "But only volunteers who pass | this filter..." controls at the top of the Announcements pane.  In the Edit Filter window, click on "Jobs". In the Job filter click on the "Add" button and choose the job or jobs you want included, and click "OK".

Type out or select the email you'd like these volunteers to receive, then hit the "Preview" button and send.