To join, see the instructions at https://www.jiscmail.ac.uk/help/subscribers/faq.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
See https://www.jiscmail.ac.uk/cgi-bin/webadmin?A0=HPC-SIG
Schedulers and related matters
HPC-SIG-SCHEDULING@JISCMAIL.AC.UK
See https://www.jiscmail.ac.uk/cgi-bin/webadmin?A0=HPC-SIG-SCHEDULING
Slack
There is a slack channel with a number of sub-channels for general issues, benchmarking, Tier 2 discussions, etc.