First thing to do is check out the listings here at Wikipedia.
Next I recommend going to your local book store (the bigger the better) and picking up a copy of Tropical Fish Hobbiest. In the back are pages of club listings and events!
You can also try doing a Google search on fish/aquarium clubs.
If you're in the Washington DC/Maryland/Virginia region, please visit the <ahref="http://pvas.com/index.htm">Potomac Valley Aquarium Society</a> for more information on our monthly meetings (next one is this coming Monday night). We're also having our big Spring All-Day Auction this April 15th!