Welcome!
Absolutely and, as Emma says, you'll get most out of it by putting most into it.
Take a look on the
groups page - some are more lively than others, I got most technical stuff out of Intensive Critique group, but the Flash Fiction group is also a lot of fun and has weekly(-ish) flash contests (from which I've had a couple accepted by www.everydayfiction.com
). The commercial short story groups are also good if you want to sell anything you've written - some refreshingly frank comments, all very useful.
From the point-of-view of etiquette, we work on the principle of reciprocal critting - you will get more people willing to crit your work if you actively comment in return. This is formally stated for Intensive Critique which I think is one of the reasons it works so well. It applies everywhere, though, so don't be shy.
When you're a full member, it's easy to chop and change, too. I'm not certain that the joining e-mail form is ever read by anyone, so I tend to join when I have a need and keep going until that need has passed.
It can sometimes appear a tad quiet, but there is a lot of high-quality advice here for the asking and I think I've always had valuable, expert answers to anything I've needed to know.
G