The main developer of MMT is Florian Rabe.
Static sources of help:
Interactive sources of help: (i.e. with humans answering)
recommended for newcomers: The MMT channel on our MatterMost instance. All questions related to MMT are welcome — from end users to API developers, from broad to very specific. With your GitHub account, you can easily join: at the previous link, first click on GitLab redirecting you to our internal GitLab instance https://gl.kwarc.info, there click on GitHub, and finally confirm this action in the appearing dialog.
Among others, we also have a dedicated channel for Math Formalizations in MMT. Feel free to join such rooms at our MatterMost instance.
StackOverflow with mmt tag (great for pin-pointed questions of general interest, e.g. recurring beginner questions)
In case you don’t get an answer there, feel free to remind us by sending a link to the StackOverflow question on the MMT MatterMost channel above.
Mailing lists (great for longer “big picture” questions)
Consult UniFormal/MMT on GitHub. Users are encouraged to submit issues for bug reports, feature requests, etc.