I have a folder in a library which I have shared with member of our staff. The invitation sent from SharePoint 2013 was received and connection, authentication and synchronization occured without a problem.
I gave access to an external user (with a hotmail account). The link in the invitation showed the folder to be accessed but authentication failed.
I am a member of the administrators group for our Sharepoint 2013 installation, but I am unable to find any reference in site settings to "allowing external users".
Anybody know the "magic word"?