Tells an object to add another object to its personal Exec list.  What the
list is for depends on the class.  The return value for this method is not
explicitly defined.

One class that uses this method is modelclass.  a modelclass object
maintains a broadcast list.  When a modelclass object gets an om_notify
message, it broadcasts an om_update message about the om_notify to every
object in its broadcast list.

This method uses a custom message (defined in <intuition/classusr.h>):

    #define  opAddMember opMember
    struct opMember {
        ULONG  MethodID;     /* OM_ADDMEMBER (or OM_REMMEMBER) */
        Object *opam_Object; /* add (or remove) this object    */
    };                       /* to (from) personal list.       */

opam_Object is the object to add to the list.  A dispatcher typically
implements OM_ADDMEMBER by sending the om_addtail message to the
opam_Object object.