From the “about” sidebar on the homepage:
GroupServer is a mailing list manager: an email sent to one address is then sent on to many people. In this way it is a list server like GNU Mailman. GroupServer also has an integrated web interface, so you can use the web to post, read the archives, and administer your groups. It is a bit like Google Groups. GroupServer gives you an open source system under your control: your groups, on your site, on your hardware.
GroupServer is flexible: a single GroupServer instance powers this site, all the sites at OnlineGroups.Net, and the E-Democracy.org Forums.
GroupServer is scalable: it handles over seven thousand email messages a month, spread across multiple domains.
GroupServer is stable: it has been in production for over ten years.
GroupServer is open-source and its source code can be found on Github. Documentation is available here.
GroupServer is developed by onlinegroups.net based in Stokes Valley, New Zealand.
Additional information and materials about GroupServer: