How do I add/delete individual .xls files from .xlw workgroup of .
 

I have grouped several Excel .xls workbooks into a single .xlw workgroup
(clicking this .xlw file opens all of the selected .xls files). How do I
change the selection of .xls files included in this .xlw group file (when I
want to add or delete an individual .xls file from being included in the .xlw
group)?