I've dabbled with a few on Meetup, but my experience has been that the cheaper ones are just a bunch of people gathering in one place to take some photos, without much in the way of tuition, and the costlier ones (which can get quite costly) tend to be full on studio shoot setups for 1 to 1 or very small groups, which i don't personally feel confident enough to handle. If you find anything in between, I'd love to hear about it!
Calumet (now merged with Wex) do have some interesting open / training days once in a while, too. I'm East London and they're now very local to me since the merger!