WordPress MU

Hi,
Working with a one month old version of WPMU.

I have another website/application that we have integrated into an office software for patient management. We have written an API to connect our office software with our website/application so that when a new user account (patient record) is created it automatically creates a user account in our website/application and sends the user their login and password.

We want to create a similar API for a WPMU install so that a user account is automatically created on the blog.

Can anyone point me in the right direction?

Thanks,
Pat