Hi,
If I understand your use case, you want to build your own visitor experience application and integrate it in the back with Aruba Instant, which are managed by Central.
One thing maybe upfront, Central with the base Device Management license primarily manages all capabilities the Instant has as a standalone product. So you can check if you could build your workflow with Instant alone and then if you can abstract this via Central and its API.
Essentially the users you create are stored in the running config of the IAP. Important: this user database is limited to 512 entries: https://www.arubanetworks.com/techdocs/Instant_654_Webhelp/InstantWebHelp.htm#UG_files/Authentication/UserManagement/ConflocalDBUser.htm
You might want to check if you want to leverage the Cloud Guest App on Central which provides Captive Portal and User Management from the Cloud and has direct APIs that you can integrate with:
http://help.central.arubanetworks.com/2.4.4/documentation/online_help/content/public_cloud/applications/guest_access/overview.htm
It's an extra license but removes many of the limitations that the internal guest portal of Instant was not designed to perform.