To enable RadContextMenu for every grid data row you should do:
1) Handle RowLoaded event for the grid and create RadContextMenu:
void RadGridView1_RowLoaded(object sender, RowLoadedEventArgs e)
{
if (!(e.Row is GridViewHeaderRow) && !(e.Row is GridViewNewRow))
{
RadContextMenu rowContextMenu = new RadContextMenu(); // create menu
StyleManager.SetTheme(rowContextMenu, StyleManager.GetTheme(RadGridView1)); // set menu Theme
// create menu items
rowContextMenu.Items.Add(new RadMenuItem() { Header = "Show row ID property value" });
rowContextMenu.Items.Add(new RadMenuItem() { Header = "Item2", IsEnabled = false });
rowContextMenu.Items.Add(new RadMenuItem() { Header = "Item3", IsEnabled = false });
// ...