Watch a Sending an Email on behalf of a Google Group (to reflect the group's email address.)
To send as a Google Group reflecting the groups email address, you will want to follow these steps:
1. Open the group from which you want to send the email and select New Topic
2. Create a new post and select the option "Make an Announcement"
3. In the email's sent by section, select the drop down menu and choose to post on behalf of your group
3. In the CC section, add in the users to which you are wanting to send to on behalf of the Group
4. Create your email and send.