How do I see the location of all of the cards in a group?

If you know which card is the parent card of a group, seeing the location of all of its children is as simple as clicking through to its card back, where the children will be shown in the Children section, grouped under their columns from the board:

However, if all you have is one of the children, and you'd like to see what its siblings are, you can click on the child to get its card back, navigate to the parent, and look at the Children section of the parent, where the card you initially clicked will be shown with its siblings.