VB.NET add local user to local group

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VB

ASP.NET

I have a VB.net class (asp.net project) for managing local user accounts.
I support more than 200 servers across north america that are not in a domain (long story).

My has no issue adding users, setting passwords and even creating groups, but I cant figure out how to add users to a group.
Here is the add user function - properties are used for gathering information - this works well

Public Function AddUser()
        Dim obDirEntry As DirectoryEntry = Nothing
        Try
            obDirEntry = New DirectoryEntry("WinNT://" & pServer)
            Dim entries As DirectoryEntries = obDirEntry.Children
            Dim obUser As DirectoryEntry = entries.Add(pUserName, "User")
            obUser.Properties("FullName").Add(pFullName)
            obUser.Properties("Description").Add(pDescription)
            Dim obRet As Object = obUser.Invoke("SetPassword", pPassword)
            obUser.CommitChanges()
            obDirEntry.Close()
            Return True
        Catch ex As Exception
            'Trace.Warn(ex.Message)
            Return ex.Message
        End Try
    End Function

Here is my function for adding a user to a group - The msgboxes are there to gather error info

Public Function AddToGroup()
    Try
        Dim obDirEntry As New DirectoryEntry("WinNT://" & pServer & ",computer")
        Dim obUser As DirectoryEntry = obDirEntry.Children.Find(pUserName, "user")
        Dim obGroup As DirectoryEntry = obDirEntry.Children.Find("Administrators", "group")
        obGroup.Invoke("Add", New Object() {obUser.Path.ToString})
        obGroup.CommitChanges()
        obDirEntry.Close()
        Return True

    Catch ex As Exception
        MsgBox(ex.Message)
        MsgBox(ex.InnerException.Message)
        Return False
    End Try
End Function

Here are the two errors

1) Exception has been thrown by the target of an invocation.

2) A memeber could not be added to or removed from the local group because the member does not exist


I have also added to other msgbox's to look at the paths returned and this is the results

obdirentry = WinNT://10.106.3.220,computer

obUser = WinNT://workgroup/10.106.3.220/test88 ------
test88 is a valid user on the target system

Any help would be great... Thanks in advance