The Select Groups window allows you select as many groups as you like. It is displayed when you set the For drop-down list to Selected... in the 
Guest Chart or 
Stationery panes.
Unselected groups are shown in the left list. Selected groups are show in the right list.
Click Add> to add the group(s) currently selected in the left list to your selection.
Click Add all>> to add all group(s) to your selection.
Click <Remove to remove the group(s) currently selected in the right list from your selection.
Click <<Remove all to remove all groups(s) from your selection.
You 
+click and/or 
+click to select multiple groups. You can also double click a group to move it from one list to another.
Click OK to accept the changes and close the window.
Click Cancel to discard the changes.