Solved! Go to Solution.
To modify the domain credentials used by ArcGIS for Server to gain access to the list of users and/or roles, you need to first log into ArcGIS Server Manager and click the Security tab at the top. Once there, click the pencil icon next to "Configuration Settings" which will open the Security Configuration Wizard. Click the Next button twice and you'll be taken to the UI for inputting the domain user account in question. Generally speaking, it is not best practice to use a traditional "user" account on the domain that is assigned to a specific person. I would suggest using something like a domain service account, which is one that is generic in name and does not fall under certain periodic password change policies.