We\'re creating an XML API for Joomla that allows partner sites to create new accounts for their users on our website.
We\'ve created a standalone PHP script that
You should use Joomla internal classes, like JUser, since there a lot of internal logic such as password salting. Create a custom script that uses the values from the API request and saves the users in the database using methods from Joomla User Classes.
Two ways to add joomla users using your custom code is a wonderful tutorial. The approach works. I've used this approach in some projects.
If you have to access Joomla Framework outside Joomla, check this resource instead.