This is despite the fact that you may have a list of groups visible in your Contacts app - Groups that you have set up on your Mac, in Outlook, in Gmail, etc. You just can’t select the group itself in the ‘To’, ‘Cc’ or 'Bcc' field.
I was asked this week if there is a work-around for this. The best work around I can offer is as follows ...
Find an email that you have previously sent to the required group of people (by looking in your Sent mailbox). Select this email, and choose the option to ‘Send Again’.
This will pre-populate the ‘To’, ‘Cc’ and ‘Bcc’ fields with the list of people.
You will then just have to remove the subject and replace with the new subject, and remove the text in the body of the email and replace it with your new text. This can be achieved quite quickly by tapping on the text to get the 'Select All' option, then just 'backspacing' to remove all selected text.
Here's hoping that iOS 8 provides this long-awaiting Mail functionality!