If you want to use SignatureSatori only for a part of your userbase, you can achieve that very quickly by following the instructions in this Google help article.
All users will still be loaded to SignatureSatori, but since we won't have full access to them, it won't be possible for you to set them a signature (the process will end in an error, and the credits you'd have spent will be returned).
The app access is managed on an Organizational unit (OU) basis.
The admins must be members of the OU with enabled access in order to log into SignatureSatori.