I can't add a new user to my organization. I'm told the user already exists, but I don't see him in my list.

If you receive an error message telling you that the user already exists, this means that he or she is already a Pronotif user through another organization. 

Contact this person to ask them to delete their account (ask them to remember to save a copy of the proof reports for shipments made to their existing account). Once this has been done, you can invite this person to join your organization.