It's pretty easy for anyone to join, provided you can get the necessary FidoNet compatable software together and running. The biggest thing is a compatable mailer. I personally use BinkleyTerm, others use ones like FrontDoor, MainDoor, PCBoard, Intermail, etc. The mailer works for you as the interface to our telephone dialup network, it makes and receives calls to and from other FidoNet "nodes" to pickup and deliver mail and file packets. It knows what numbers to call and who is who by using a nodelist.
This nodelist is a listing of the 30,000 FidoNet nodes worldwide and their telephone numbers and information about their modems and software that might help you connect to them. Most times you won't even need to run the full nodelist since most of the mail is routed via Hubs and centralized mail processing machines and Internet ftp connections to Regional Star Hubs and on to it's destination. The nodelist comes in a raw text file that has to be compiled into a usable indexed database for the mailer to use so that it can do fast lookups of the people you need to call.
Another important part of FidoNet software is the mail processor. In fidonet we use a simple but powerful packet stucture that has a binary header affixed to help in the routing, it says where it came from and where it needs to go, etc. The mail processing software uses this header information to look at incoming packets and "toss" the mail into the apropriate areas on your BBS and to send what needs to go somewhere else to the apropriate nodes in a process sometimes called "packing". You may already be using software that can handle FidoNet compatable packets, some BBS packages do. Also, since most BBS systems use a semi-standard message base system (not all do though) there is usually a FidoNet compatable tosser that will handle your specific BBS message base, and work with the incoming and outgoing mail.
Once you get all this stuff working and can set up your system to be available for FidoNet mail sessions (an hour a day is reserved exclusively for these session, and you can also do them in between your BBS calls as well) you'll need to contact the Net Coordinator in your area and apply for a node number. There is some rules called Policy4 you'll need to familiarize yourself with, and then agree to abide by. They also describe the application process. Dallas is net 124, and the Coordinator is 124/0 (currently Frank Vest), and you'll need to apply via a FidoNet Netmail message to the NC. A period of testing will follow while the NC determines if you meet the minimum requirements of a node. If you make it through the testing (mostly their system will call you and make sure it works and is compatable with FidoNet and they'll ask you some questions to make sure you understand Policy4), you'll get a node number and get connected to a Hub, and be on your way.