If you're interested in saving money from SugarSync, here's how to use Sierra's new iCloud Drive's ability to store your Documents folder on it and to share that same Documents folder (identical and synced between computers) among multiple computers.
So, now I'm operating with two Macs who are using the same, identical documents folder on my iCloud drive. From the above that I've applied, here's how to do it:
1. Use SugarSync to make sure the Documents file on each of your computers is synced and the same. If you're not on SugarSync, there's a 30-day trial you could take advantage of just for this task. This syncing of your Documents files by SugarSync will eliminate your need to one-by-one, file-by-file make sure that the two computers all have the same files, and that one doesn't have some files that the other doesn't.
2. Once this is done, quit SugarSync on both computers, or even uninstall SugarSync on both of them. You can always reinstall SugarSync by downloading SugarSync again from the SugarSync website. Google "uninstall SugarSync" for directions to do this. This is important (to quit both or uninstall both). If SugarSync is running while you begin transferring your computers' Documents files to your iCloud Drive, SugarSync will start syncing your sync and cause a mess, potentially.
3. On one computer, keep all your documents and files in your Documents folder (do nothing).
4. On each of the other computers that you want to be sharing the same Documents file (your second computer and third, etc., not the one on which you've done nothing, per #3), copy all the files and folders in each computer's Documents folder to an external hard disk, flash drive, or (if you have room) to your computer's user folder. Just make a new folder, title it "Documents Folder Backup," then drag all your folders and files from your Documents folder to this folder. This is just a backup that can be temporary. The result is that you should have one computer (your first one) with all the files and folders in its Documents folder and all the other computers with Documents folder that are completely empty. The reason you do this is the following: if you don't, your iCloud Drive will automatically form a subfolder in your iCloud Documents folder for your second and third and fourth computers using iCloud Drive with the "Documents and Desktop" box checked. Hence you'll have two of everything on your iCloud Drive and your files will not sync. If you want to keep the Documents folders from two computers unique (not the same, but different), just leave all your folders and files in your second and third etc. computers, and you'll have separate, nonsyncing Documents folders on your iCloud Drive that will be subfolders of your iCloud Drive's Documents folder. Again and in contrast, if your second and third computers have empty Documents folders before doing the next step, then their Documents folder will be identical to the first computer's and all your files will sync across all your devices/computers whenever you make a change on one.
5. Next, on the first computer--the one with all your documents in its Documents folder--do what Leonie has said, above: go to System Preferences>iCloud and click Options for iCloud drive, and turn on Desktop and Documents Folders. Once you do this, this computer's Documents folder will begin to upload to your iCloud Drive into a newly created folder on your iCloud Drive named "Documents." This may take a long time, if your Documents folder on your computer has a lot of files. Wait till this completes. Note: it may seem like nothing's happening for five minutes. Be patient! Pretty soon, your computer's Documents file on your side bar will go from the top to lower down underneath the "iCloud" segment, and you'll see a pie-chart/clock showing you the progress of the uploading. If you click on the iCloud Drive Documents folder in your side bar, you'll start to see all your Documents folder's files and folders start to be listed and then actually uploaded. Wait till this is all done. It may take a few hours.
6. When this is done, you will have your Documents folder both on your iCloud Drive and resident and saved on your computer (although that won't be intuitively apparent that it's also on your computer's hard drive, but turn off your wifi and open and use the file to prove it to yourself). Thus, when you're out of wifi, because the files are in both places (one your hard drive and your iCloud Drive), you'll be able to use every file from your hard drive. Your computer just won't upload the edited or newly created file to your iCloud Drive till you're later back in wifi.
7. Go to your next computer, one with the empty Documents folder, and repeat: go to System Preferences>iCloud and click Options for iCloud drive, and turn on Desktop and Documents Folders. Once you do this, this computer's Documents folder will drop down in your sidebar to the iCloud Drive section, as with your first computer, and it will begin to populate with all the files and folders that were formerly in your Documents folder on this computer (that you've backed up). These files and folders will be coming from the upload of the Documents folder from your first computer, which has now completed (in step 5). This may take a few hours. The result will be that you have saved copies of all these files on computer #2, and that as you work with any of these files or create new files, they will be saved on your computer #2's hard drive, uploaded and saved on your iCloud Drive, and will change or create the file on your first computer, too. Your first computer will have the newly edited file saved on its hard drive automatically. Everything will sync and be identical across the two computers.
8. If you have a third or fourth computer, just repeat step 7 for each, one at a time.
9. Once you're confident in a few weeks that this is working as you'd like it to, you can:
9a. Cancel SugarSync and probably save a bunch (I only need about 30 GB of storage, so I'm going from SugarSync's $79? per year to Apple's $12 per year.
9b. Delete your Documents folder backup that you made in step four, unless you just want another backup.
Thanks to Leonie for all the help!
