Hello,
Check the friendly name in the following Identities key - HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Common\Identity on the users machines. Make sure that is correct
and matches their display name.
If that does not resolve your issue I would recommend opening a support ticket so we can troubleshoot the issue further.
You can open a support ticket by going to
http://support2.microsoft.com/gp/offerprophone/en-us?ln=en-us&x=7&y=11
and clicking on the Get Started Button.
Thanks,