There is not currently a way on Market to filter for private groups.
I'd recommend the following:
-Host your apk on a private server
-Have a URL on said server where the app can ping to check the version number of the latest available apk.
When newer version is found:
-Download it to specific location using DownloadManager.
-Throw a message in the user's notification bar using NotificationManager
-Set it up so that when user clicks on that notification, an install prompt for that apk file appears. Details for that are in another SO thread.
