Humanist Organizations
Some good organizations are:
The American Humanist Association
American Atheists
The Humanist Society of America
Council for Secular Humanism
Wikipedia listing of a good list of a number of US humanist organizations
In addition there are a number of local and regional groups that you should be
able to find on any search engine. They can have a number of activities you may
find useful and can be a source of support when needed.
The Greater Worcester (MA) Humanists have the following page as a good
example:
http://www.meetup.com/worcesterhumanists/
Scroll down their page to see the various activities they have