What happens when a non-member of a site is invited to a project?

This depends on the site configuration. If the 'Allow User Join' option (In ‘Domain Configuration’ page) is enabled, the invitation is sent to the invitees. There is a link present in the email notification that is sent. This link points to the login page from where the user can register himself to the site. On the other hand, if the 'Allow User Join' option is disabled, the following error is displayed: << There is no user registered with an email address of "user@domain". Either use a different email address or contact your administrator to have an account created for the user you want to invite.>>