I was published in book form a few years back, but I didn't really come into the project the way you're talking about (submitting ideas); I was a member of an open-source writing team. We created the original PHP 3 manual online and then carried on with PHP 4. At one point, a fellow dropped me a line asking if I'd like to help author a book on the language, so we worked it out with the publisher and I came on board. Happily at the time I had a nice severance payout from the dot com I worked at in Finland so I was able to take a year off to write before looking for another job. Without that I don't think I'd have had the time.
But like I mentioned, I just kind of happened to be in the right place at the right time there, so that doesn't really help you.
I have often considered submitting to magazines and like Ron said, there are various ways to go about it. Some have information in the masthead about what they're looking for; some have a page or article entitled "Request for Papers" or similar; some have an address you can write to for a writer's packet, and so on. Oh yeah: check the website, too.
As a teen I was planning an article for a magazine and wrote in for a writer's kit. I got a manila envelope containing various stuff like an unsigned contract, a description of what they were looking for and a fairly comprehensive list of what they considered acceptable, what they'd pay for various things, and so on. Sadly I never completed the article so I can't tell you how it plays out.
My feeling is that most niche magazines welcome submissions.
As far as local groups go, find a web page or someone affiliated with the group and just walk up and ask them. With electronic publishing, just try checking out the Contacts page or emailing an admin. They may not be able to help directly, but they can probably point you in the right direction.
Good luck,
Torben