Perhaps you can use the mailman contrib, this is a package used for managing mailing lists. Although it is more geared to exchanging mail from all members in the group to all other members, perhaps you might be able to configure it that some people can post to it, and others can only read from it. For mailman please see the wiki, IIRC there is a page on it in the wiki.
I am not sure if what you are planning to do is legal. In many countries it is required to opt-in for mailing lists and you are not allowed to subscribe users by harvesting their e-mail addresses from your servers logfile.