Civilized discussion for your community
Discourse is super flexible open source forum software. It’s a top Community Tool.
The private company behind Discourse is Civilized Discourse Construction Kit, Inc.. They offer paid hosting for forums starting at $100/month.
You can apply for free hosting for open source projects
Discourse for Teams
There is a new (late 2020) Discourse for Teams offering that starts at $20/month for up to 5 team members on a fully private forum.
Discourse for Teams is a private, focused version of Discourse with special tools to enhance remote work, productivity and internal discussions.
Self-hosting Discourse is quite reasonable to do on a ~$10 month VPS. The instructions and updates are very well done to be automated, and you can just cut-and-paste commands. It uses Docker, plus a pretty simply process to add additional plugins and re-run the Docker process.
There are instructions for running a second Docker instance to enable Discourse email posting in an integrated way as well.