RadGrid Get Selected Row Index from Item Template Button
<telerik:RadGrid ID="RadGrid1" runat="server" AllowFilteringByColumn="True"
    DataSourceID="cusGrid" GridLines="None" Skin="Default" AllowPaging="True" DataKeyValue="CustomerID"
    PageSize="500" AllowMultiRowSelection="True" ShowStatusBar="true" >
        <MasterTableView AutoGenerateColumns="False" DataKeyNames="CustomerID" DataSourceID="cusGrid">
            <RowIndicatorColumn>
                <HeaderStyle Width="20px"></HeaderStyle>
            </RowIndicatorColumn>
            <ExpandCollapseColumn>
                <HeaderStyle Width="20px"></HeaderStyle>
            </ExpandCollapseColumn>
            <Columns>
                <telerik:GridTemplateColumn UniqueName="CheckBoxTemplateColumn">
                    <ItemTemplate>
                        <asp:Button runat="server" Text="Select" OnClick="SelRecord" />
                    </ItemTemplate>
                </telerik:GridTemplateColumn>
    ...

protected void SelRecord(object sender, EventArgs e)
{
    var gRow = (GridViewRow)(sender as Control).Parent.Parent;
    var key = string.Empty;
    if (gRow != null) { key = gRow.Cells[0].Text; }
}

protected void RadGrid1_ItemDataBound(object sender, GridItemEventArgs e)
{
    if (e.Item is GridDataItem)
    {
        GridDataItem dataItem = e.Item as GridDataItem;

                int selectedRowIndex = dataItem.RowIndex;
    }
}

<asp:Button ID="Button1" runat="server" CommandArgument='<%#Container.ItemIndex%>' OnCommand="Button1_Command" ... />

protected void Button1_Command(object sender, CommandEventArgs e)
{
    GridDataItem item = RadGrid1.Items[(int)e.CommandArgument];
}

foreach (GridDataItem item in RadGrid1.SelectedItems)
        {
            GridDataItem item = (GridDataItem)RadGrid1.SelectedItems;
            var key = string.Empty;
            key = item.ItemIndex;
        }

var gRow = ((sender as Button).NamingContainer as GridItem).Selected;

if (gRow != null) { key = gRow.Cells[0].Text; }