Try this website for starters:
http://www.bicycletexas.com/
On that site, you'll notice a lot of the listings show a 2008 date. Those rides may or may not be happening this year. Try clicking on the link and see if they show an updated ride date on the group's actual website.
Local bike clubs oftentimes have a listing of area rides, and bike shops may have fliers for them, too.
I think they just had a big Katy ride. Up here in Dallas, people generally run all over the metroplex and beyond for these rides. So I've been to Paris, Sulphur Springs, Tyler, Granbury, etc. Most of the listings on that site are the kind where you pay your $30 and go ride, which is the kind I like.
I've been riding a "non drop" ride at a local bike shop, and that's something else you might check into. "Non drop" supposedly means that if you're slower, they won't run off and leave you. From other people's posts, I gather that's not always the way it works, but you might check into it anyway. With the group I ride with, the faster people run on ahead, and wait up somewhere.