A question I often get while delivering the BPOS courses for partners in Belgium and Luxemburg is how to collaborate with external users on SharePoint sites. In BPOS (Business Productivity Online Suite), you can only grant access to users that have an account in the customer’s BPOS subscription and those users consume a license. Although you can use a deskless worker license, you still need to provision the account, manage it and pay a monthly fee for it.
In Office 365, it will be possible to grant external users access by using their e-mail address. The only caveat here is that the external user has to logon with a Live ID that matches the e-mail address you granted access to. If they do not have one, they will have to create one. Although I would have liked to see support for Open ID, Facebook Connect or other systems this new possibility will make it much easier to collaborate with external users.
The SharePoint Online Administration Guide (still beta of course) that is now available on TechNet documents the process in two sections: