I have used following PS Script to add a user to a group on a web. I didn't any error but I cannot find the user in the group or anywhere else. Thanks for your advice.
New-SPUser "wm\test2" -Web "http://mydomain/sites/s1" -Group "reader on s1"
Try this: Set-SPUser -Identity 'fun/factory' -Web http://someserver -Group 'Name of group'
I think you need to do a web.Update after adding a user.
(I'm not a machine with SharePoint installed at the moment to test, sorry)
What happens when you try to get the user in PowerShell?
Get-SPUser "wm\test2" –Web "http://mydomain/sites/s1"
Furthermore, does the user already exist in your domain?