If you want to view all images that are contained by a single parent folder: Right click on the parent folder and click "Open folder and subfolders in new/replacing contact sheet"
If you want to view various folders that aren't contained in a single parent folder, open a folder as a contact sheet, then go to the additional folder in the navigator that you'd like to see added to the current contact sheet and either hold option (on Mac) and double click on that folder. On Windows I believe you'll want to hold down the Shift key.
