Hey Andy,
You could call the [SCIM API](http://www.simplecloud.info/) from your existing registration page to add new users to the server. We currently support SCIM 1.1, but will be adding support for SCIM 2.0 in the next release of the server. [Here's the documentation](http://www.gluu.org/docs/admin-guide/user-management/#scim-oxauth-authentication) for using SCIM with the Gluu Server. Feel free to open another ticket if you have any specific questions.
Thanks,
Will