Symptoms

Adding multiple list items with People or Group column consecutively throws this error:
The user or group USER] could not be found
 

Diagnoses

There have been cases where this happens on a multi-domain environment wherein either a user is being returned more than once or not at all due to issues communicating with the AD.
 

Resolution

On Workspace, there is an option to ignore foreign principals by ticking the checkbox: Management Console > User Managers > K2 > Settings. This was followed by a service restart.