It depends on where you are in the grid. If you have Library/Show Photos in Subfolders enabled and you're in a parent folder, you can't move images that are stored in a subfolder (and displayed in the parent folder). If you're in the subfolder you can click and drag images to rearrange them. You can click and drag in a standard collection, but not in a smart collection. One simple way to do it is to create a collection name it slideshow or something, set it as the target collection and then add images for the slideshow to that collection using the Painter tool, Shift-B, by right-clicking and selecting Add to Target Collection or by clicking in the top-right of the image (not the image frame).
HTH,
Mike.