Start a new email. On the menu toolbar, click on View and All headers. This put the BCC (blind carbon copy) header above the Subject box on your email.
Click on the address book next to BCC and select your group name from the list which appears. Make sure you add the group name to the BCC box on the next screen, by clicking on the BCC -> button in the middle of the screen,
Click on Ok and then send the email as normal.
Post again if this doesn't work.