Hi Jonathan,
I'm sorry to hear that you've experienced an error when trying to add a user to your Adobe Sign Account.
Generally the error occurs when the email address you are looking to add:
1) Already has an active Adobe Sign license
2) Has trialled the Adobe Sign service in the past
3) Or has eSigned a document before using Adobe Sign
If an active license exists, we cannot add this user to a trial Account. If a trial was used in the past or a document was eSigned before using the service, we can add the user on your behalf.
I'll send you a direct message shortly to investigate this further, and add the user on your behalf in case of 2) or 3).
Hope this helps!
Kind regards,