The Container needs to be cast to Infragistics.Web.UI.TemplateContainer. You can either store in the CommandArgument a unique identifier of the row, like a data key for example (here DataItem should be used)

CommandArgument=<%# DataBinder.Eval(((Infragistics.Web.UI.TemplateContainer)Container).DataItem, "PostId") %>

or reference the actual underlying object. In case of the ItemTemplate it is the cell. To get the row index, something like that can be done (here the Item property should be used):

CommandArgument=<%# DataBinder.Eval(((Infragistics.Web.UI.TemplateContainer)Container).Item, "Row.Index") %>

This latter approach however has a problem, the Item property is currently referencing the same object as the DataItem. It's been fixed and the fix is coming up shortly.

I wil try this.  I tried the below and it worked on the first page of the grid. On later pages, the Command Argument gets cleared out somewhere, even though I see the correct value set in wdgNotes_InitializeRow for rows on the later pages, just like the first page.  By the time it gets to the wdgNotes_ItemCommand on pages after the first page, the Command Argument is an empty string rather than the value it originally held.

protected void wdgNotes_InitializeRow(object sender, Infragistics.Web.UI.GridControls.RowEventArgs e)
    {
        TextBox tbNote = (TextBox)e.Row.Items[0].FindControl("tbNote");
        tbNote.Text = tbNote.Text.Replace("~", "\r\n");

        Button btnEditNote = (Button)e.Row.Items[6].FindControl("btnEditNote");

        string sGroupID = (string)(((DataRowView)e.Row.DataItem)["GroupID"]);
        //CheckSecurity();
        // Turn off Edit Note button if they do not have Read/Write or Read/Change access or
        // if they are in a different group than who created the note.
        if (((sAccess == "RW") || (sAccess == "RC")) && (sGroupID == sEmpGroup))
        {
            btnEditNote.Enabled = true;
            btnEditNote.CommandArgument = e.Row.Index.ToString();
        }
        else
        {
            btnEditNote.Enabled = false;
        }
    }

protected void wdgNotes_ItemCommand(object sender, Infragistics.Web.UI.GridControls.HandleCommandEventArgs e)
    {
        Button btnEditNote = sender as Button;
        if (btnEditNote != null)
        {
            //CheckSecurity();
            // If they press the Edit note button, pull up the Edit Note popup window.
            //  The security check is redundant, but an extra check does not hurt.
            if ((btnEditNote.CommandName == "Edit Note") && ((sAccess == "RW") || (sAccess == "RC")))
            {
                int intRowIndex = Convert.ToInt16(btnEditNote.CommandArgument);
                DataRowView drvRow = (DataRowView)(wdgNotes.Rows[intRowIndex].DataItem);
                tbNoteToAddEdit.Text = ((string)(drvRow["Note"])).Replace("~", "\r\n");
                dtbCreatedDate.Visible = true;
                lblCreatedDate.Visible = true;
                dtbCreatedDate.DateValue = (DateTime)(drvRow["CreatedDate"]);
                ViewState["NoteIdentifier"] = (string)(drvRow["Identifier"]);
                ViewState["Private"] = (string)(drvRow["Private"]);
                ViewState["GroupID"] = (string)(drvRow["GroupID"]);
                lblAddEditHeader.Text = sConstEditNote;
                btnAddEditNotePopup.Text = "Update";
                AddEditNotePopupWindow.Show();
            }
        }
    }

Yep.  The current version won't let you reference Row.Index.

DataBinding: 'System.Data.DataRowView' does not contain a property with the name 'Row'.

Unfortunately, I need then to keep a DateTime field as a command argument.  It then loses the milliseconds.  I can see them in the grid, but the command argument loses them.  Any suggestions?

CommandArgument='<%# DataBinder.Eval(((Infragistics.Web.UI.TemplateContainer)Container).DataItem, "CreatedDate") %>' />

Maybe doing something like that will help?

CommandArgument='<%# DataBinder.Eval(((Infragistics.Web.UI.TemplateContainer)Container).DataItem, "CreatedDate") %>.<%# DataBinder.Eval(((Infragistics.Web.UI.TemplateContainer)Container).DataItem, "CreatedDate.Milliseconds") %>'