When trying to add a user to a dash board the user does not populate as an available user to add.
In one scenario, the user was not an active user in the type of Report evaluated (ex: Salesforce).
When you create a Dashboard / Widget (example, Investigate) you start with basic attributes. However, as you add Filters (example: Activity Type per image below), the activity types may only apply to certain records/apps (such as certain Securlets). So even if the user populates for other apps/Securlets, etc. adding a specific filter can make it so the user doesn't appear in the list.
• Make sure the user that is trying to be added is an active user in the SaaS
• Try editing/removing Filters to open the scope since applying specific filters/activity types may not be applicable to your Dashboard/Widget..