If you have an FEA Project, you can use the graphics window to create new groups.
Group > Create group using graphics > Grid stiffened panel..
You may make a desired Unstiffened group active and change the view so you are only seeing that single group. Then hold [Shift] + [Alt] and draw a bounding box to select all the components in view. Repeat this process for each unstiffened group you wish to change to a grid stiffened.
Then, to delete the empty unstiffened groups you can use the "delete range of groups" option.
Group > Delete range of groups...
If you have a Non-FEA project, you can use the utilities spreadsheet to automate the group creation and the component membership.
http://hypersizer.com/help/index.php#COM/XLS/com-xls-wks-setup_groups.phpLet me know if this helps.
-James