Honestly, Lightroom doesn't seem like the best choice is these situations.
You could import from each folder, if the option to not import duplicates is checked, then you only get the new photos. This is a manual solution.
You could set up a watch folder in Lightroom, and new photos in there will be automatically imported, but that only scans the ONE watch folder.
If people are moving photos from folder to folder, this will cause the problem in Lightroom where the file is not found, and there seems to be no way to avoid this other than to prevent people from doing this, or go to a keyword based organization instead of a folder based organization.
We have a network location for our stock photo library where various people will store their downloaded stock photos in folders and subfolders. I use Lightroom classic to catalog the photos. I have imported all of the photos into a catalog. My question: Is there a way to periodically have lightroom scan those folders for new photos that have been added and add them to the catalog? Or even a manual way to update the catalog with newly added files from those folders? People are also moving photos from one folder to another and I keep ending up with "file not found"
If you have a hierarchy with one top folder, then manually using 'Synchronize Folder' on that top folder will add all new photos to the catalog. However, if people move photos as well, then Lightroom will suggest to delete the old and now 'missing' photo (i.e. the photo in the old location) from the catalog and add the 'new' photo (i.e. the photo in the new location) to the catalog. That will cause all edits and other added metadata (like keywords) to get lost. Maybe you should consider using Bridge rather than Lightroom.
That's it! Thanks. At first I was confused by your answer thinking Syncronized folder and a watch folder were the same thing. But a watch folder just handles single photos and copies them elswhere which would not work. Right clicking on the top folder and choosign "synchronize" does the trick - it updates with new photos and removes any that have been moved. The metadata is not important here, we don't do much editing and adding metadata. Most of these photos are set up for others in the company to find clip art easily for powerpoints and such. I am the only one even using lightroom to track the files.
I wish Adobe would have a setting to do this automatically, but manually is fine for now.