Just to clarify the method that should work using the Google web address
https://mail.google.com/mail/?tab=wm :-
- You Hit Compose to create new message,
- You then Click on the 'To:' text that Opens which displays the Contacts Page,
- You then click 'My Contacts' to expand the list were you should now see your previously created / imported Group name(s).
- If so, clicking on the group name should display the list of contacts contained within and you can then click 'Select All' or select each address individually.
The above works for me.