I can't find any reference anywhere on how to create a user via API for Rackspace Cloudfiles. I take it it is not possible? Is there any equivalent function to Amazon AIM API?
It's possible.
You would need to use the Rackspace Identity Service API. In particular, have a look at the Users resource API. If you want to restrict those users to a product like Cloud Files you'll need to use the Roles resource API.
It will also be helpful to read up on the Rackspace Role Based Access Control. You'll need to know the Permissions Matrix for RBAC if you're assigning roles to your users.