To join, see the instructions at https://jiscmail.ac.uk/help/subscribers/faq1.html and the individual links noted below. The email lists are curated by the HPC-SIG secretary, but you should register yourself, and then will be accepted if valid. The secretary does not ordinarily add members themself.
Main list
HPC-SIG@JISCMAIL.AC.UK – subscribe via https://www.jiscmail.ac.uk/cgi-bin/webadmin?A0=HPC-SIG
Schedulers and related matters
HPC-SIG-SCHEDULING@JISCMAIL.AC.UK – subscribe via https://www.jiscmail.ac.uk/cgi-bin/webadmin?A0=HPC-SIG-SCHEDULING
Slack
There is a slack workspace with a number of channels for general issues, benchmarking, Tier 2 discussions, etc. It is regularly used to discuss issues, ask questions and advertise jobs. Visit hpc-sig.slack.com to sign up.