Hey @jimmend
The Invite users button in the Netlify UI doesn’t use that endpoint. Check out [Support Guide] Understanding and using Netlify's API which states:
The most important advice I can give you about using our API is to watch how we do it! Our entire admin UI works via this API…
This is an undocumented endpoint though (from what I can tell.) It may or may not work for you (now), and may or may not change in the future.