Please check out: <a href="https://groups.io" rel="nofollow">https://groups.io</a>. We support markdown/code syntax highlighting, Github integration, member sync with Slack, and lots more.
NodeBB[0] and Discourse[1] are good alternatives... They have a lot of integrations including github and syntax highlighting<p>[0] <a href="https://github.com/NodeBB/NodeBB" rel="nofollow">https://github.com/NodeBB/NodeBB</a><p>[1] <a href="http://discourse.org" rel="nofollow">http://discourse.org</a>
I recently moved a google group to Google Plus Communities[0]. It's a free option and all your community members already have a google login.<p>[0] <a href="https://plus.google.com/communities" rel="nofollow">https://plus.google.com/communities</a>
This wouldn't work for larger projects, but if it's a smaller project, maybe Slack[0]? It has a pretty good free-tier as well.<p>[0]: <a href="https://slack.com/" rel="nofollow">https://slack.com/</a>
Discourse maybe a good choice: <a href="http://www.discourse.org/" rel="nofollow">http://www.discourse.org/</a><p>You will need to pay to host it though.