The main developer of MMT is Florian Rabe.
Currently substantial contributions are being made by Dennis Müller and Tom Wiesing. Moreover, various members and students of the kwarc research group work on individual projects relating to MMT.
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)
Communication between developers and users as well as among users. Announcements about MMT (e.g., new releases) as well as any general requests or inquiries.
Communication between developments. More technical.
Consult UniFormal/MMT on GitHub. Users are encouraged to submit issues for bug reports, feature requests, etc.