For some lists, membership is restricted via an IT Department controlled alias.
For others, all you need to do is visit the following link, replacing "4CUG" with the list you want:
https://listserv.cccnext.net/scripts/wa-cccnext.exe?SUBED1=4CUG&A=1
Or you can start with that link and it'll give you a drop-down option showing all available lists.
More instructions can be found here.