This is the preferred way for asynchronous communication. The forum is hosted by Google Groups.
IRC is great for synchronous communication.
#mayocat on freenode.net. Here is a direct link to a web client. Developers and contributors will usually be available between 08:00 and 21:00 GMT.
You can also get in touch with the makers of Mayocat directly.
You can report issues you encounter with Mayocat Shop on our GitHub project page.
Committers (developers with committership responsibility) will gladly study pull requests submitted to the project on GitHub. If you provide the project with several quality patches, and are interested in leveling up to the next stage of responsibility, you can be voted as a committer on the project.