This method adds a gadget to the group object's list.  The group object
will increase the size of its select box to include the new gadget's
select box.  The group object moves the new member to an absolute location
(by changing the new member's GA_Left and GA_Top attributes) relative to
the group object's upper-left corner.  Note that all members of the
groupgclass object will be deleted by OM_DISPOSE.