NotifyObservableCollectionChanged

I have a situation where I have to marry some old and new code. I have an older POCO class that receives data items via HTTP request. It receives and stores these items in JSON format for offline use. There is no problems with the implementation and it works as designed.
I have a request to add a new "view" of this data that requires me to implement a "grouped" list of these items. I have created an "ItemByGroup" class with the group name and a list of subsequent items in each group. The Linq Query places these items into the new grouped list and works as designed and is repsonsive...
I now need to be notified if the original list of items changes and subsequently update the "ItemsByGroup" so the secondary view is current and accurate.
I have been trying to find a reasonable example of using "NotifyObservableCollectionChanged" so if the original list changes, I can have it update the "ItemsByGroup" through that notification stream.
Can anyone provide assistance on this?