You can only delete a group if you created it. To delete a group you created: 

1. Head to the People page by tapping on the people icon in the bottom right of the screen 
2. Choose the group you want to delete 
3. Once viewing the group, tap on the group options icon in the top right of the screen 
4. Tap 'Delete group'. Please note that this will delete the group for all members, but will not delete the group's upcoming events.