JMP will paste into all selected cells for a selected column whatever value is in the clipboard. Therefore, if you create a new column and then copy into your clipboard "Group A" and you then rt. mouse click on the top of the Rowstate column in JMP and select Row Selection, you can use a variety of different tools to select the rows you want to have the "Group A" , then just go to the selected area of the new column, for one of the selected rows, and paste into that cell, the paste will happen for all selected rows.
If you want to get a little more fancy, a feature of JMP Scripting Language(JSL) is that you can assign a value to all selected rows for a given column. The statement that has the same functionality as my above example is:
newcolumnname[data table("the table name")<<get selected rows] = "Group A";
So it would be a very simple matter to develop a JMP Addin that when run would ask the user for what column to target, and what value to place in that column and then when OK is selected, the input value would be placed into all selected rows for the column specified.
Jim