I ran into an issue at work where we couldn’t add a user to a local computer group. I later determined that you couldn’t find the user anytime you had to use the “Select Users, Contacts, Computers …” search window, like when you add the user to an AD group.
You could, however, find the user in the “Find Users, Contacts, and Groups” search box in ADUC.
I used a Powershell command to output the properties of the user account to a text file. I did the same thing for another non-problematic account. Then I used Beyond Compare to compare them. I found that the showInAdvancedViewOnly attribute was set to TRUE on the problem account and FALSE on the non-problem account.
Import-Module ActiveDirectory Get-ADUser ProblemUser -Properties * | fl c:\temp\problemuser.txt
I changed the value of showInAdvancedViewOnly to FALSE and now I can find the account and add it to groups.