hey all, the plugin can be downloaded at https://wordpress.org/extend/plugins/buddypress-announce-group/ (once they approve it)
but for the time being it can be found at http://bluemandala.com/plugins/bp-announce-group.zip
it is pretty straight forward and can work with public groups, private or even hidden groups. Basically it make is to that only admins and moderators can add content. Regular members can join the group but can’t add any content.