The Austin polyamory group has about 680 members right now. Austin is a drive from Ft. Worth, but it may be worth it. I would also check meetup dot com.
I believe that the DFW area has a poly group. They may not be an active yahoo group, as the Austin group it, but a DFW group does exist. In fact, I met a woman on PolyMatchMaker from that area. We talked on the phone after exchanging a few emails, but we've went our seperate ways now.
Update: Here's the link to the DFW poly group on yahoo.
http://groups.yahoo.com/group/DFW-Poly/