Jazz-L is a mailing list for people who like jazz. Since the first day I became involved in it has consistantly included some extremely knowledgable people among its members. In general, however, the denizens of Jazz-L do not feel constrained to speak only about jazz; frequently some of the most interesting, and certainly some of the more entertaining, threads of discussion have had little, if any, relationship to jazz.
Not everyone enjoys this approach, which is fair enough. However, you should not join with the expectation that the group will be 100% on-topic with respect to jazz.
If you join up and find that you are in the middle of some trippy discussion in which half the participants are speaking in pidgin english and claiming to be ghosts, criticizing this behavior will probably not get you good results. Instead, I recommend leading by example: If you come up with something interesting to talk about, or have something neat to say that no-one else has thought of, people will generally recognize that fact and follow your lead. (Your mileage may vary, natch.)
There's a semi-official Jazz-L homepage:
If you've made it this far and are still interested in checking out Jazz-L, here are the instructions on how to subscribe:
Send mail to listserv@brownvm.brown.edu with the following line in the body of the message:
sub jazz-l firstname lastname
(of course your full name should be substituted for firstname lastname).
After you successfully subscribe, LISTSERV should send you back a welcoming message. Hold onto it, as it contains instructions on how to get yourself off the mailing list if and when that becomes an issue (if you find that it's not to your interest, or you switch internet providers, or you're going on vacation for a while and don't want to come back to a jam-packed mailbox).
I guess that's it. If you want to get ahold of the owner of the jazz-l mailing list (currently me, but of course that could change) send mail to jazz-l-request@brownvm.brown.edu.