The Group feature ("Arrange" > "Group" or Ctrl+G) lets you combine multiple photos and their respective edits into a photo group which can be selected and edited as a single object.
Grouping photos is useful in the following scenarios:
- Clone tool
- Filters (e.g. red-eye)
- Mask tools / selection tools
- Color editing or color removal
Grouped photos can be ungrouped by going to "Arrange" > "Ungroup" (Ctrl+U).
You don't have to ungroup objects to edit them individually, however. Each object in the group can be individually selected by holding down the Ctrl key and clicking on the object or by selecting the object in the Page & Layer gallery.