Haraka has an active Mailing List which you can join by sending a mail to email@example.com. We are working on providing list archives.
The core developers have much of their discussions about features in the #haraka IRC channel on irc.freenode.net. It is highly recommended to join the IRC channel if you are new to Haraka and wish to ask questions. They will be answered fastest there.
For everything else, we use Github. Post bug reports there on the issue tracker and feel free to browse the code and try and learn the structure of the project.
Haraka can be found at haraka/Haraka on Github.