If you're trying to join this community, and you receive the error message, "A user with this username already exists." you'll have to change the user name to something not already used.
For example, if you try to join, and you get this message:
Then, try making the name unique. You might just add a number to the user name. In my example, I changed "TroyWolbrink" to "TroyWolbrink2" and it worked: