In order to leave a Private Group from an iPhone, follow these steps:
1. From the private group map, tap the menu icon in the top right of the screen which will take you to the details of your group.
2. Here you can swipe left over your information and tap leave group.
3. You will then be asked if you want to leave the group and you should then tap Leave.
4. Finally, you will be taken back to the main screen and a notification will be displayed letting you know you are no longer part of group (name_of_your_group).