I believe it's a problem with my forum and not the settings I chose...he's the first person I invited to register, and apparently both his username and email were already taken? Weird...

Is it just his username, or email also? Forumotion filters out simple usernames as "spam" to prevent any spam accounts. Tell them to try a different username that's either longer/complex to see if that works. You can later change his username through the admin panel, or he can if the option is already open for use under profiles.

Yes, your nickname can't been shorter then 3 letters, and some simple nicknames forum use like spamming, and you can't register with them. And you can see if you have censore that name. I'm on my forum having same problem. Tell him he try to use some "hard" nickname, and you will change that through admin panel.

Hi,

Check the following settings: ACP >> General >> Forum >> Security >> Unauthorize subscriptions made with the most commonly used spam usernames. : [] Make sure it's on "NO."

Regards,
Sir Chivas.

Also check here:

Administration Panel > Users & Usergroups > Users > Inactive List