property Group.UserData as Variant
Specifies an extra data.

TypeDescription
Variant A Variant that indicates the group's extra data 

The UserData property associates an extra data to the item. The UserData property is not used by the control. The UserData are of Variant type, so you will be able to save here what ever you want: numbers, objects, strings, and so on. Use the RemoveGroup event to release any extra data associated to the group, if case.