That was the question I received the other day in a private message in Office 365 forums. Fortunately, the answer to the question is positive and as a proof, I’m sharing two screenshots that show how an external user can sync a team site document library using OneDrive for Business sync client:
First, navigate to the SharePoint Online site and log in there using an external user that has access to it (remember that an external users needs to be identified by a Windows Live ID and he also has to be granted access to the site).
Then, just go to the document library the external user has to sync and simply verify the “sync” link is enabled for him. One last thing to remind here is that in order to have the “sync” link enabled, the external user needs to have installed the OneDrive for Business sync client that is completely free and can be get from the following support url: http://support.microsoft.com/kb/2903984/en-us