Some of this documentation is also available off-line in Mantid, via Help -> Mantid Plot.
Please report any bugs you find, so that we can keep improving Mantid. The easiest way is to contact the development team in the CS group, especially for any issues that are causing major problems.
There is also an option to report bugs in Mantid, via Help -> Report a Bug, but please only use this for minor bugs. This link allows bugs to be posted on the forums. Alternatively you can use the contact form or email mantid-help@mantidproject.org.
There are Mantid user meetings arranged approximately once a year, for instrument scientists and facility users to meet and discuss the direction of the Mantid project. The location switches between the partner facilities, with the next of these tentatively planned for January 2019 at the ILL.
The user forums provide an easy way to report problems, ask questions and get news. You can sign-up for an account at http://forum.mantidproject.org/.
There is also a mailing list for Mantid announcements, such as releases and user workshops. Usually there is about one email per month. You can sign-up at http://lists.mantidproject.org/mailman/listinfo/mantid-announce.