BuddyPress Group Email plugin adds group email functionality to BuddyPress. Once installed, a new item is added to the group menu allowing any group member to send an email to all the other members in the group.
Features:
Easily communicate with group members without having to locate and input all of their email addresses
Ensure that messages are received, even if a user’s notifications are turned off
Keep users interacting on your site with an in-house communications center for their various groups
More info: http://premium.wpmudev.org/project/buddypress-group-email