Finding new users in NWC

I'm trying to create a workflow for new employees to read and acknowledge some company policies, preferably on their first day of employment. I have the workflow set up and everything works fine, except for one thing.

The workflow is designed so that an HR employee inaugurates it, by using a People field to select the employees who need to be assigned the task. No need to describe all the details of the workflow but, as I say, it all works fine in testing when I select my own name (as if I were a new employee).

But here's the problem: NWC can't find new employees using the People function. We have some new employees starting today, as I write this, and when I try to find any of them with the People field, it tells me "No content available."

I've encountered this issue before when trying to assign ownership to a workflow, and it seems that the users have to log in to NWC at least once before a People field will find them. When I saw it before, I just had to ask the person to log in. But I don't want to do that here. I want the new employees simply to receive the assign-a-task email with the link to the content they need to read and acknowledge.

So my question is: is there a way to "force" NWC to find new users without the users having to take any action? We already have SSO set up from NWC to our Active Directory, so I'm wondering if there is some setting or something that can be changed to make this happen. (I am not an admin in either NWC or Active Directory, but I can pass along any information I'm given.)

Thanks for any help that can be provided.