Our mailinglist firstname.lastname@example.org is used for just about anything.
Feel free to ask questions about pdfgrep here, as well as report bugs or send patches! It will also contain official announcements and is generally very low traffic.
There is also an archive of all old messages.
Bugtracker and Pull Requests
If you’d rather use a bugtracker or create pull requests, then the project page on GitLab is for you.