BuddyPress.org

Don’t think that’s possible at the moment and I don’t know of any plugin that does it.

Have a look at the Group Extension API and these functions:
`messages_new_message` – Let’s you send a message to one or more members
`groups_get_group_members` – Gets all group members (there’s functions for group mods and admins as well)

With these functions and the API you can build a contact form where group admins can contact their members.