Hey Jørgen,
I'll do an international answer here for posterity...
First (about the trouble with using the create-user api method; returning an invalid signature error even when the signature was input correctly), we had a bug on our end miscalculating the correct signature for the method. The bug has been resolved now, so you should be able to create users through the api.
Hopefully, this will allow you to set the password etc for the users you're creating.
Secondly, users cannot be deleted entire for security purposes. This means that you won't be able to reuse the same email address and username over and over again for testing; but a trick is to user pseudo mail address such as steffen+1@23company.com and steffen+2@23company.com which will end up in the same mailbox, but be considered different by 23 Video's API.
Thirdly, information about existing users is available through http://www.yourdomain.com/js/users (in JSON format)