AFAIK there is no API for multisite invitations. You will require to create your custom endpoint.
Your question refers to a website hosted at wordpress.COM. However, you are here in the forum of wordpress.ORG. The difference is explained here, for example:
What’s the difference between WordPress.org and WordPress.com?
Please contact the support of wordpress.COM with your request: https://wordpress.com/support/
The .com API you linked to looks like a normal .org users API that’s oriented for multisite. The .org version I believe returns users for the current site, so you’d need to construct your API URL to reflect desired site.
Neither API is specifically for invited users AFAICT. It’s for any registered users. It’s likely you could discern invited users by certain user meta data.
