You can stop sharing to a Private Group from an iOS device in one of two ways:
1. Open the navigation menu in the upper left corner.
2. Select the private group.
3. On the group map screen, either select yourself on the right, or select the beacon indicating you are sharing to the group.
4. Depending on the option you chose, find Stop Sharing, and select it.
5. You will receive confirmation that you are no longer sharing to the group.