I finally joined the Romantic Novelists' Association for the first time this year. I could have joined (as a full member) over forty years ago when my first novel was published but at the time didn't see any real advantages of joining. With a full-time job and a young family, I couldn't have gone to any of their meetings or conferences. Even now, I feel more like a nominal member because my membership consists solely of receiving the RNA magazine three times a year. Maybe if I had more time, I would be more pro-active.
Apart from that, the only things I have joined are several yahoo groups. I don't contribute as much as I feel as I should, but then if I contributed regularly to them all, I wouldn't have any time to do my own writing.