Navigate to the Users page.  Click the filter icon.

Expanding this filter allows for new ways to sort and communicate with volunteers.  This example would allow for emailing all users within a 'Corporate Groups' user group who volunteered for an event within the 'cashier' event group from January first through the month of April. 

Once the options are configured, press the filter again to display the list of users.  You can also select your desired communication method (Text or Email) to reach out to those volunteers!