To get the selected items of a data grid, you can use the SelectedItems property, which will give you each of the selected items in your collection.
For example, if your DataStore is a collection of DataRow objects, then you can do:
foreach (var row in myGrid.SelectedItems.Cast<DataRow>())
{
var id = row["user_id"];
// do something with the id
}
Cheers!
Curtis.