How do I get Reading Users from a NTFS group ( Active directory) via vb.net

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VB

I want to read all members from a certain NTFS group. So far I have this code, but it doesn't work. (It works just to get the names ( name& surname)) I want to get the usernames

Dim dsGrupi As DataSet = GetGrupi()
Dim db As New dbGate
For i = 0 To dsGrupi.Tables(0).Rows.Count - 1

    search.Filter = [String].Format("(cn={0})", dsGrupi.Tables(0).Rows(i)(0).Trim())
    search.PropertiesToLoad.Add("samaccountname")
    search.PropertiesToLoad.Add("member")
    search.PropertiesToLoad.Add("description")
    result = search.FindOne()

    If result IsNot Nothing Then
        Dim dn As [String]
        Dim clenovi As [String] = ""
        Dim equalsIndex As Integer
        Dim commaIndex As Integer
        Dim user As String = ""

        For counter As Integer = 0 To result.Properties("samaccountname").Count - 1
            dn = result.Properties("samaccountname")(counter).ToString()
            equalsIndex = dn.IndexOf("=", 1)
            commaIndex = dn.IndexOf(",", 1)
            user += dn.Substring(equalsIndex + 1, commaIndex - equalsIndex - 1) + ";"

        Next

        txtUsers.Text = user

    Else
        MessageBox.Show("Group doesnt exists!")
    End If

The error I get is this: Index was out of range. Must be non-negative and less than the size of the collection.