Danny Burlage posted External users in SharePoint Online 2010 #office365 on 11/15/2010:
SharePoint Online allows you to collaborate with users outside of your organization by giving them permission to access your internal organization resources. These users are called external users. If the site collection administrator or site owner wants to allow external users to access a site, the SharePoint Online Administrator must first enable the access in the SharePoint Administration Center this can be done when you set up a site collection or after it is set up. After the ability to invite external users is set up, the site collection administrator can invite authenticated external users to access the site via e-mail. The external user then accesses the site by using Windows Live ID user name and password.
Support partners are business and technical professionals, including Microsoft, who provide support services to Microsoft Online Services users. By default, the Microsoft Online Services administrator grants support partners access to the SharePoint Online Administrator Center from the Office 365 portal. However, if you want a partner to have access to a site collection or site, you must explicitly grant the partner access. You can do this at any time by changing the setting on the site collection in the SharePoint Online Administrator Center.
This new concept which will be launched in SharePoint Online 2010, part of Office 365, will enhance the capabilities for granular corporate governance to great extent. We are really happy with this feature because it will allow us to manage environments of our customers without them needing to worry about us getting access to all their data.
I expect customers to be really excited about this feature because it will allow them to give access to both partners and customers in a manner which is closely controllable.