Users
The Users table is populated by downloading a list of Docusign Users from Docusign and allows you to specify a custom Sender when using the sending envelopes on behalf of another user. You download the list of Users from the Users tab on the Account Details screen - see the Accounts guide for more information.
If there have been changes to Docusign User you can download the latest details for the Docusign User record by clicking the Update User from Docusign button at the top of the User Details layout:

You can also Authenticate a User to generate their Access Token when using the Send on Behalf Of feature with the JWT Grant - see the Sending Envelope Signing Requests On Behalf of Another User guide for further details.