You can stop sharing to a Private Group from an iOS device in one of two ways from your Private Group map:
1. Navigate to the 'User Details' by tapping on your user and tap on 'Stop Sharing'
2. Select the Beacon, indicating that you are actively sharing location, at the top of the map. You are then taken to a page showing details about your Glympse where you can then tap 'Stop Sharing'.
Once you have completed either of these steps, you should be taken back to the original Private Group map where you will see that you are no longer sharing location.