First, who is "Us"?
GNU Mailman is free software, developed and used by people from all over the world. We work on Mailman as volunteers and give it away for free. There is no official paid support organization behind Mailman; we are a community.
That said, there are some official channels for contacting the folks involved with Mailman.
We have a steering committee of long-time core developers who collectively make most of the global project-based decisions. If you'd like to contact us please email mailman-cabal at python dot org
If you would like to report a security issue, or have other security related questions, please email mailman-security at python dot org.
Most of the developers, and lots of users, also hang out on the #mailman channel on the freenode IRC network.