SigningHub for SharePoint automatically integrates all your Active Directory users with your SigningHub enterprise account. The Active Directory users are self-registered under the default role of your enterprise, when they perform any SigningHub related activity (i.e. Share a document, Sign a document, etc.) from their SharePoint account.

For this you just need to ensure that:

+ Your Active Directory users have been made the part of SharePoint site groups where the SigningHub add-in has been installed, and 

+ The SharePoint site groups have been assigned the right permission levels.

These users can then easily log into SigningHub by using their MS Active Directory credentials, learn how the MS Active Directory users can log into SigningHub.   

For the SharePoint users to access the SigningHub for SharePoint add-in, their "Work Email" should exist in the user profiles of Active Directory and SharePoint both, and it must be the same. The users which do not have their "Work Email", will not be listed under the "SigningHub Users Access" screen.