You can stop sharing to a Private Group from an Android device from your Private Group map by selecting the Beacon (blinking arrow), indicating that you are actively sharing location, at the top of the map. You will then be taken to a page showing details about your Glympse where you can tap 'Stop Sharing'.
Once you stop sharing, you are back in the Private Group map where you will notice the Beacon is no longer blinking, indicating that you are no longer sharing location.
