I have a SharePoint 2016 on-prem server farm that I manage. It is set up to sync users from Active Directory and everything appears to be working fine. Since there are multiple OUs containing accounts that I don't need in SharePoint, I originally set the profile sync up to use a limited set of OUs. Recently, we got a request to set up a site that would be shared with users in a different OU that I hadn't imported. I went into the user profile service and modified the AD connector so that it would bring the users in. After performing another sync, I checked and the user profiles are now visible and searchable. However, when I go into People and Groups and attempt to add one of these users into a group, they are not available. Any idea what I'm missing here?
The People picker doesn’t extract user from User Profile service application, it extracts user information from User Information list and Active Directory.
Try to navigate to Central Administration->Monitoring->Review Job Definitions, find the “User Profile to SharePoint Quick Sync” and “User Profile to SharePoint Full Sync” timer jobs, run them and make sure they are succeeded, check the result.