By default, uninvited users are not allowed to register and join a delegated site.  To enable uninvited users to join a site (also known as user self-registration), go to the Site Settings page of the delegated site and click Delegation Settings under the Delegation Configuration menu.

On the Delegation Settings page, select Yes or No in the Registration Settings section to either allow or deny uninvited users.  Click the Finish button to apply the new setting.