Congrats, you are the president of the new San Antonio Bills Backers Chapter.
It's easy to find members. Just officially register the chapter at bb.com, and as the contact person, you'll start getting emails from bb.com with the contact information for people who try and register for your chapter. Another way is to put a small ad in the local shopper with something like "Attn: Buffalo Bills Fans" as the title. Most of those ads are usually free. Also something else I did was create some business cards for the chapter with your name, email, Phone, ect., and put them on the counters at local sports shops, card shops, bars, or anywhere's else you think a football fan will go.
You want to designate a meeting place to watxh games, usually sports bars are the ideal place, but it can be your backyard, it's totally up to you. After this, the sky is the limit, keep in contact with your members, make raffles for Bills merchandise, do charity events, put together road trip packages for your members.
Good Luck!