To be fair, it is difficult to achieve "full understanding" with such garbage documentation as DocuSign's.
DocuSign has a legal obligation here to prove authenticity, how are they ever going to be able to do that if everything is behind a single account?
They support oauth and that makes sense and should be the way to do it.
I had a similar experience. I explored their API and got stuck on how to implement my use-case and how to ensure it's legally binding. Sales and "technical" resources assured me it was possible, didn't explain how, and everyone balked at any sort of legal questions and basically told me that was all on us to sort out.
I decided I didn't need help creating a box for users to scribble on. E-signature isn't a technical challenge at all.
By the signature.
I don't know how it works in DocuSign's internals, but there's no requirement for the signer to have an account. The point of the account is for users to see all of their documents in one place. In OP's case that's everyone's documents because they use a single account.
https://support.docusign.com/articles/How-do-I-sign-a-DocuSi...