What is the default Joomla 2.5 user registration url?
The default url for Joomla 2.5 to access the user registration page form without creating a menu link is:
index.php?option=com_users&view=registration
Just add that string after yourwebsitename.com
For example:
http://www.yourwebsitename.com/index.php?option=com_users&view=registration