Mailing list to ping all admins at once: email@example.com. Note that this list is private but anyone can send an email to it without sign-up.
Gitter channel Scikit-HEP/community for informal exchanges.
Certain packages recommend that general questions be asked with relevant StackOverflow tags when the README does not immediately provide an answer. The available package tags are:
You can also get in touch via the GitHub pages of the package of interest, e.g. to issue a bug.