Step One: Form the group
Before you do this, check and see if there are any existing groups that you can join or incorporate.
Meetup.com allows for customization of the group’s site, great calendar coordination and as soon as you start the group it automatically emails all those in the area who are interested. It is free to join and look for groups, however starting a group costs $72 every six months. That isn’t too bad as long as you take dues to spread that cost out.
Facebook also has a number of advantages for starting a group. The first of which is that it is free. Secondly, nearly everyone has a Facebook account, so it is very easy for people to find the group and share it with other non-believers. Unless you’re really ideologically opposed to Facebook, you should have one even if you have a Meetup account.
Step Two: Find members
If you go with Meetup, they’ll do the first round of recruiting for you. Next you should go to: http://www.atheistnexus.org/profiles/members/advancedSearch
and look for people in the area. Let them know the group exists. Do the same for your area on Facebook. Become a bit of a Facebook stalker by searching for area freethinkers by using a myriad of terms in the religion category (non-believer, anti-theist, none, no thanks, ect.). Also, post flyers at libraries, coffee shops and universities.
Step Three: Schedule your first meeting
Your first meeting will probably be small, but remember that these groups grow quickly. Start with something simple like a discussion at a coffee shop. Get to know your new members.
Step Four: Organization
Once you’ve been meeting for a while, set up official bylaws for your group. This helps the group stay focused and provides the structure for elections, installing a board of directors and generally making sure that the group doesn’t rely on just you to keep going.
Here is an example from the Atheist Community of Austin: www.atheist-community.org/history/archive/constitution.pdf
Step Five: Get listed
Getting your new group listed in directories will give more people access to you as well as make it easier for speakers to contact you and set up talks.
American Atheists: http://atheists.org/
Council for Secular Humanism: http://www.secularhumanism.org/index.php?section=main&page=alg
Drinking Skeptically: http://drinkingskeptically.org/index.htm
Skeptics in the Pub: http://www.skepticsinthepub.net/Find_a_Group/
Grassroots Skeptics: http://ohioskeptic.com/grassrootsskeptics/
Atheists Alliance International: http://www.atheistalliance.org/programs/alliance/affiliates
Starting up a Student Group
There are far more resources for campus groups: