J. J. wrote: I created a shared album. I realize the photos others have added are fairly low resolution and lack some metadata, but I'd still like to add some of them to my library.
Is there any secret to this? Or is it just a matter of selecting the photos and dragging them to "Library" in the side bar on Photos for Mac?
I don't have any added pictures, so I can't try it-- but dragging them to a regular album (or to Library) is what I would do. In any case, there's no harm in trying it.
These pictures in a Shared Album are kind of "elsewhere" in iCloud rather than in your Library space, so they don't use any of your own iCloud space. Of course, when you drag them to your own albums will take a bit of space-- probably not much since, as you say, they are lower resolution.