This is how I managed to do it on the HTC Incredibe II:
Under "People" select the group icon at the bottom of the screen.
Select the group that you want to email
Press the 3 arrow "share" button
Click "group mail button" button
On my Samsung Galaxy Nexus (OS 4.1.2) I can't see how to email a group via contacts, because there doesn't appear to be any sharing option. The Contact GroupsU app works ok for emailing one group, but you can't include several groups in the one email (as far as I can see). On the occasions when I want to email several groups in the one email, I just use the desktop version of Gmail on my mobile and this works fine. Except that I found I had to use the older version of Gmail, not the newer one with the pop-out email window, which only allowed me to send to one group. It also lets me send via BCC, so the recipients don't see all the other addresses. Limited re attachments, can only attach an image from your photos.
I think that this is a needed feature for the GMail app on Android. They have passed the app off as being almost identical to the desktop version, but for mobile. Yet, a feature that screams efficient productivity is left out. No one wants to type out individual email addresses when composing an email to a group. This feature has long been a staple in GMail on desktops. It just makes no sense that this feature is not supported, especially for those of us who have ICS on our phones and have no clue as to why there are groups in our people app yet we can't send anything to the group.
Can anyone answer why the GMail app does not support groups? I think it's time we got down the bottom of this.
This is not a complete solution, but it helps me just to type it once, copy all the emails and paste them into a memo, which I keep as a widget on my phone. I then copy and paste the emails when I want to send to the group. It's a pain, but it saves a little time
No clue as to why Groups are not integrated into contacts, HOWEVER, I devised my own workaround. In the full-blown Gmail, create Draft emails with your chosen Group name.......you can even make it a BCC:..........then, on your tablet or android cellphone, select the Draft folder. Here is your stash of Group emails to use. From here, you can edit, include attachments etc., & send it to your Group. Ofcourse, you'll have to create several drafts so that you always have spares.