I belong to several.
Mystery Writers of America. I joined long ago when my first book was published--and it wasn't even a mystery. I've maintained my membership through the years. And to be honest, I've kept it mostly because of the prestige associated with it. I've never lived close enough to either the Southern or Northern California chapters to attend any of the events.
Sisters in Crime. I joined the national group year ago, before there were many chapters. I get a lot of information from being on the listserve.
I went to the first get-together where a group of mystery writers gathered together in Fresno to talk about organizing a chapter--now the San Joaquin SinC. I've belonged ever since and do attend meetings whenever I can. I've gotten a lot out of these meetings, besides making many, many friends and hearing lots of interesting speakers, buying lot of author's books, I also got some ideas for books from speakers who were in law enforcement and a dandy idea from a coroner.
I belong to the L.A. chapter of Sisters in Crime so I could participate in the L.A. Times Book Festival. I've only attended a few meetings because it's so far away, but I do enjoy the newsletter and being on the listserve.
And I'm a member of the Central Coast chapter of Sisters in Crime and have been for a long time. I have many good friends from that group, and hubby and I love going to the coast so join them for a couple of their events a year.
I've been a member of Epic for many, many years, way back when no one understood or really knew anything about ebooks. I've been to many Epicons and not only learned a lot but had a great time attending. Unfortunately, air travel has become difficult for me and probably will not got to many more.
PSWA (Public Safety Writers Association) is a favorite because the group is mainly made up of many types of law enforcement and other public safety people who write--and a few of us mystery writers who like to pick their brains and hang out with them. Their listserve is wonderful, a place where you can ask all sorts of questions and get many answers. I was the program chair for the conference for many years, now I'm the newsletter editor.
And probably the most helpful is the critique group that I belong too. This group is invaluable to me and what I consider to be my first editor. Of course they've all become friends too.
Now, you answer my questions and be sure to tell me why you belong to whatever groups.