RF subgroup creation

I am at EE7.9.4 now. In Expedition PCB RF application, there is a Group/Ungroup function allows us to add group and add subgroup manually in interface. I would like to automate the add group and add subgroup using script below. Unfortunately, I can't find a way to automate "add subgroup". Any idea?

Dim GroupColl : Set GroupColl = docObj.FindGroup("MainGroup")

Dim GroupObj

If GroupColl Is Nothing Then

    Dim MainGroup : Set MainGroup = docObj.PutGroup(epcbGroupRF,"MainGroup")

Else

  For Each GroupObj In docObj.Groups(epcbSelectAll, epcbGroupAll, True)

    If GroupObj.Name = "MainGroup" Then

         Dim SubGroupColl : Set SubGroupColl = GroupObj.Groups

         If SubGroupColl.Count > 0 Then

                'How to create subsequent subgroup?

         Else

                'How to create 1st subgroup?

         End If

     End If

  Next

End If