Lumen is an open-source group discussion platform with dual web-email access.
It can be compared to Discourse, Loomio, Slack, Google Groups and Facebook Groups.

Lumen is designed to run on your own domain (e.g. example.org) or subdomain (e.g. lumen.example.org).
Here it's running at lumenapp.com. Try it out by signing up to the Previewers group.

Try Lumen

Organisations using Lumen

Get in touch for managed hosting (£50/month) or install yourself.