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.
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