User u =[SELECT Email FROM User WHERE Id IN (SELECT UserOrGroupId FROM GroupMember WHERE Group.DeveloperName ='CaseQueue')];
String us = u.Email;
 Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
 String[] toAddresses = new String[] {us};
 mail.setToAddresses(toAddresses );
 mail.setSubject('No Users available to handle this case');
 mail.plainTextBody='handle this case';
 Messaging.SendEmail(new Messaging.SingleEmailMessage[] {mail});