The ikiwiki project strongly encourages collaboration through ikiwiki itself, and thus does not have a mailing list. Anyone can create an account on ikiwiki's own wiki. ikiwiki provides a bug tracker, a TODO list, and "discussion" sub-pages for every page, as well as a forum for general questions and discussion. ikiwiki developers monitor RecentChanges closely, via the webpage, email, and IRC, and respond in a timely fashion.
You could also drop by the IRC channel
However, if you find a new security vulnerability, please email the maintainers privately instead of raising it in a public medium, so that we can arrange for coordinated disclosure when a fix is available. The maintainers are Joey Hess (firstname.lastname@example.org), Simon McVittie (email@example.com) and Amitai Schleier (firstname.lastname@example.org).