; Create an SD data set and get the Reference number:  
file_id = HDF_OPEN('demo.hdf', /ALL)  
SDinterface_id = HDF_SD_START('demo.hdf', /RDWR)  
dim=[100]  
SDdataset_id = HDF_SD_CREATE(SDinterface_id, 'demo_data', dim, 
/FLOAT)  
ref = HDF_SD_IDTOREF(SDdataset_id)  
HDF_SD_ADDDATA, SDdataset_id, FINDGEN(100)/10.45 + 2.98  
HDF_SD_ENDACCESS, SDdataset_id  
HDF_SD_END, SDinterface_id  
  
; Use the Reference number to add the SD to a Vgroup:  
SD_TAG = 720  
vgID = HDF_VG_GETID(file_id,-1)  
vg_handle = HDF_VG_ATTACH(file_id, vgID, /WRITE)  
HDF_VG_SETINFO, vg_handle, name='data1', class='demo'  
HDF_VG_ADDTR, vg_handle, SD_TAG, ref  
  
; Use HDF_VG_INQTR to verify the SD was added correctly:  
IF HDF_VG_INQTR(vg_handle, SD_TAG, ref) THEN $  
   PRINT, 'SUCCESS' ELSE PRINT, 'Failure'  
HDF_VG_DETACH, vg_handle  
HDF_CLOSE, file_id  

IDL Output

SUCCESS