UPDATE: We no longer have the meetup site but we're still around. Our website is http://freethoughtoasis.org/
for the dfw area most of the active groups are members of the DFW Coalition of Reason www.dfwcor.org There are a lot more groups than just dallas/plano and frisco. I am one of the directors of Fellowship of Freethought which is headquartered in Dallas.
Houston has at least two groups, San Antonio has two, Corpus Cristi has one, Austin has at least one, there are at least two in East Texas (not on meetup) and then there is the odd scattered middle of nowhere group. Most of the main college campuses have groups.
If you can't find atheist groups search for other main labels such as secular, bright, freethinker, freethought etc.
btw, I'd highly suggest anyone trying to grow or maintain a group at least get their group a page on A/N as well as facebook even if not paying for meetup...no one is going to know the group exists if it is not on the internet in some sort of social media group.