Sending Message To Group members
Design and Skin Customization
Only the newsletter system:
your-site.com/letter/compose
But it divides on quota, not groups. Are you thinking from the point of view of the group owner being able to send messages, or the admin user.
Seams a little open to abuse if you let the group owner who is not the site admin send emails from your site.
The feature would need to be added, it doesn't exist right now.