When you can’t find a user in the list, maybe this means that they are disabled.
You can check this and enable the user again by going to Users > Users and click on the button View Disabled Users in the top right corner.
Check if you can find the user in the list, click on the three dots, and choose Enable.
Now the user is enabled (and visible) again. Click on View Enabled Users to go back to the active users.
When you enable a user, they will receive an automatically generated e-mail.
You can change this template in Settings > Communication > Mail Templates. The mail is called User Reenabled. Don’t forget to check the translations as well.
If you don’t want this mail to be sent out automatically, please contact support at firstname.lastname@example.org or via the webchat to disable it for your platform.