The 
Change Group window allows you to move one or more guests from one group to another. To display this window select one or more guests and then select Edit>Change Group....
Move guest(s) displays the guests that will be moved.
Select the group you want to move the guests to in To group and click OK or double-click on the new group.
Note You can also change the group of a guest by dragging and dropping them from one group to another in the Groups pane or by setting the 
Guests pane Mode to Edit and editing the Group column.