In the dialog that appears, make sure the path shows the correct location of the folder you want to sync with, which must have the same name (not necessarily the same location) as the one on the first Mac. If not, you can edit it or click Change and navigate to a new location. Be careful here — if you're syncing a folder named Example and you choose, say, /Users/you/Example in this dialog, BitTorrent Sync will add a folder at /Users/you/Example/Example. If you want to sync with an existing folder called Example, choose its parent folder (as in /Users/you) or edit the path manually to eliminate the duplicated folder name. (If the folder you select doesn't already exist, BitTorrent Sync creates it. If it does exist and you click OK at the "Add anyway?" prompt, the software merges the contents of the folder on the second Mac with the contents of the folder on the first one.) Click Connect.
Now go back to the first Mac, where BitTorrent Sync shows an alert asking you to approve access from the new client. Click the green check mark to approve.
And that's it — from now on, the folder you selected on each end should stay in sync across your Macs as long as they're both online. (For files over 4MB, BitTorrent Sync copies only the changed portions of files, in 4MB chunks, which speeds syncing considerably.) You can open the same email message on additional computers and click the link to add more peers, or repeat the entire process to sync additional folders.
You can also use the BitTorrent Sync iOS app (or comparable apps for other mobile devices) to view or download any of your files, although the mobile apps don't sync entire folders automatically since that could overwhelm your device's storage.
Share a folder
You can also share any of your folders with other people, either read-only or read-write. To do this, open BitTorrent Sync, highlight a folder, and click Share. Choose the options you want, which are identical to those you saw when you set up the folder for syncing, and click Email to send someone an invitation.
As is the case when syncing your own folders, each person you share with must have BitTorrent Sync installed on at least one of their device. Unlike cloud storage services, there's no website someone can connect to to see your files in a browser.
Feeling left out
The least intuitive aspect of BitTorrent Sync is how to go about excluding a file or folder from syncing when you're syncing its parent folder. For example, if I have an app that already syncs its data across Macs in some other way (DEVONthink Pro Office is one such app I use every day), letting BitTorrent Sync copy the same files would lead to conflicts and inconsistencies. So I want to tell BitTorrent Sync, "Sync everything in this folder except the following subfolders."
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