Creating a Custom Login Page for SharePoint 2010/Manage external sharing for your SharePoint online environment

http://msdn.microsoft.com/en-us/library/gg317440(v=office.14).aspx

http://office.microsoft.com/en-us/office365-sharepoint-online-enterprise-help/manage-external-sharing-for-your-sharepoint-online-environment-HA102849864.aspx

http://blogs.msdn.com/b/kaevans/archive/2010/07/09/creating-a-custom-login-page-for-sharepoint-
2010.aspx

SharePoint Online for enterprise

SharePoint Online for small business

ON by default, can be turned off SharePoint Online Administer “allows” for whole company. Individual site owners choose to activate the “external user invitation” feature for their group’s use. Site owner invites via any email using Site Actionsà Share Site. Invitee accepts invitation with a LiveID-based user name and password (@live.com, @hotmail.com, @msn.com and all regional derivatives). Collaboration ensues. Undo step 1 or 2 at any time.

OFF by default, can be turned on Site owner invites via any email using Site Actionsà Share Site. Invitee accepts invitation with a LiveID-based user name and password. (@live.com, @hotmail.com, @msn.com and all regional derivatives). Collaboration ensues. Deactivate the “external user invitation” feature at any time.

http://community.office365.com/en-us/blogs/office_365_technical_blog/archive/2011/12/08/did-you-know-you-can-share-sharepoint-online-s-external-sharing-capabilities.aspx