The #scummvm IRC channel on irc.freenode.net is also a good place to ask questions. Many ScummVM developers hang out there regularly, as well as many ScummVM users.
We offer several forums. Use the Help and Support forum if you encounter problems using ScummVM. The General discussion forum is, as the name suggests, meant for general discussion of any topics somehow related to ScummVM. Also there are some platform-specific forums and the Junkyard.
Don't forget to read Forum Rules before your first post.
If you think you found a bug, take a look at our bug tracker. Maybe a similar bug has already been filed, in which case you can add a comment to that bug report. If not, you can submit a new bug report, but please carefully read the instructions given on the submit form. And of course, follow them :-)
Feature requests should go on our feature request tracker (again first check if something similar has already been requested).
Finally, if you have made a modification to the ScummVM source code and want to see it merged back into the ScummVM main line, you can file a Pull Request on our github page for that.
There are three ScummVM related mailing lists. Two of them are for automated content only. The one where you can send emails to yourself is scummvm-devel.