We have been using the "Shared Envelopes" feature to allow specific users who are not the sender or a recipient of the envelope to be able to view an envelope in our applications without any issues. I used the following instructions to do so:

How to embed the DocuSign UI in your app: https://developers.docusign.com/docs/esign-rest-api/how-to/embed-ui/
This lays out using the SDK method Envelopes::createConsoleView.

However, DocuSign has deprecated "Shared Envelopes" in favor of their new "Shared Access". We need "Shared Envelopes" to be maintained until the API is updated to support "Shared Access".

If this is not done API developers will have no way to do a document console view in our apps anymore for "shared" users of a "service account". This would be a pretty big piece of functionality to take away from DocuSign customers.