With the new OneDrive for Business sync client you can share a folder with a co-worker. Since the January 2017 update you can also sync the shared folder to your device. This works with the Windows and Mac sync client.
A common use case for this is when you want to collaborate with a few co-workers on some documents. The owner creates a folder on his OneDrive and then can sync the folder with one or more co-workers.
The co-workers now get an e-mail with a link to the folder. When they click on that link they are redirected to the OneDrive for Business web interface. On that page you see a ‘sync’ button. A click on the button starts the OneDrive for Business sync client on the device and syncs the files on your local drive.
Note that this is a 2-way sync. When some of the co-workers delete a file, the file also disappears for the other co-workers. The file is stored in the users recycle bin where it can get restored from.