I’ve just managed to join after a lot of trying.
I’m using an iPad (iOS 13.4.1) and Safari as a browser. Very frustrating to fill out the Join form and find, after filling in all of the details that username already exists. I tried again and again, even with a user name that could not possibly exist! Then cleared the cache and tried again with the same result. Eventually I tried the same thing, with same obscure username but this time using Firefox as the web browser. I could register without problems.
Not sure if the issue is with your coding or more likely with Apple, but glad to have got through the first hurdle!