In a case like yours, that involves private messaging, you just need to add a little bit of implementation in your Metadata Adapter.

We assume that on the client side you set the user ID by this function. On the adapter side, with the NotifyUser method you can identify the user and assign him to the correct group. Then in the GetItems method you can customize the Item (i.e. "MyItem" became "MyItem_GroupA" or "MyItem_GroupB" depending on which group the user belongs).

Now you can feed the item "MyItem_GroupA", "MyItem_GroupB", ... independently with the quotes appropriately corrected and users will receive only those of their group in a transparent way.

For a further discussion on the topic of private messages, please take a look at this thread.

Hope that helps.