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.
Schedulers and related matters
There is a slack channel with a number of sub-channels for general issues, benchmarking, Tier 2 discussions, etc.