The fastest way to contact us is via the Gitter channel Scikit-HEP/community, ideal for informal exchanges. See also related rooms HSF/PyHEP and HSF/PyHEP-newcomers from the HEP Software Foundation (HSF).
Mailing list to ping all admins at once: firstname.lastname@example.org. Note that this list is private but anyone can send an email to it without sign-up.
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.