This little script will do it
names default to here(1);
dt=current data table();
:Group[dt<<get selected rows] = "Group A";
Interactively you can also do the same very quickly.
- Select the rows from clicking on the Histogram
- Right Click on the chart and select Name Selection in Column
- In the dialog window that pops up, specify the column to use, and the value to give to the selected rows
- Click on OK and the cells will be filled in
Jim