Turns Drupal into a multi-group email list.
OG Mailinglist allows users to start new discussions by email and post comments on existing discussions by simply replying to emails. You get the best of both worlds: a browser-based discussion site and an email-based list.
Suppose you have implemented Organic Groups and have three topical groups: "Web Programming", "Technical Help", and "Politics". Users normally go to the group page and click "Create Story" to start a discussion. With this module, a user simply emails "email@example.com" to start a new discussion. The subject becomes the story title, and the body becomes the story body.
Using the regular Drupal notifications framework, all users in the group get an email telling them of the new discussion. These users would normally click the link to make comments. With this module, a user wanting to post a comment simply hits Reply and sends his or her comment. The reply mail also goes to the "firstname.lastname@example.org" address.
In many ways, your Drupal site becomes a mailing list manager. Users never have to visit the site to start or take part in discussions. The module automatically detects new or deleted groups and is always up to date.
This module allows group members to post to your site directly from their email box -- it bypasses their login name and password and simply matches the email address. This can be spoofed by anyone smart enough to do it. The module has some security in place, but by its very nature, it is a potential security risk. Use this module only on sites where this is acceptable.
There are two ways of installing this module. First, the easier method, is to use Mailgun's email service to send and receive emails. OG Mailinglist ships with a sub-module which provides the necessary integration.
The second option is to setup up your own email server. This requires root access on your site's server and fairly advanced sysadmin skills.
For installation instructions see documentation.
For easier subscription there is OG Quick Join module.