Hi Nate,
I'm really sorry to be "that guy", but are you able to post the code you use to create a user via XMLRPC? I've been battling for hours trying to get the array structure correct (in PHP); it keeps throwing this back at me:
Invalid external api parameter: users (Only arrays accepted.)
Anyway, not your problem I know, I was just hoping someone could share their "create user" knowledge if they've already won the battle.
Cheers,
Dan