The event should not get generated by the ListBox but some underlying logic. Bind the ListBox to a BindingList
] instead of a List. Any changes to the list will get displayed without the Update() call.
However, you still need the method known as Update_listbox(). Just name it according to its purpose: changing your bound online users list.
And then again, you need to call it. How is it called now? Does the skype4com library provide an event?
As a last resort, there's always a Timer (or this, or that) to poll the data.