MathGroup, www.mathgroup.org, is the original Mathematica usegroup founded and moderated by Stephen Christensen. This high-quality bulletin board is a treasure trove of invaluable Mathematica know-how, and an incredibly useful resource for answering your Mathematica questions and problems. Introduction and instructions for subscribing and posting can be found from here: http://forums.wolfram.com/mathgroup/.
There is a newer Mathematica usegroup in the StackExchange group of collaboratively edited question and answer sites: http://mathematica.stackexchange.com/. Their intro says : "Welcome! This is a collaboratively edited question and answer site for users of Mathematica. It's 100% free, no registration required." The functionality of the StackExchange sites is interesting (http://stackexchange.com/sites).