Thank you for the feedback. We're actually currently working on SCIM integration with Okta, Active Directory and OneLogin to make the process of provisioning users safe and automated. If you use any of these Identity Providers and are interested in testing this new functionality, can you send me a direct message through the forum here?
Thank you all so much for your feedback and patience. We've a Public Users API in beta with user provisioning endpoints accessible using Oauth. The Public Users API will let partners, developers and customers provision users (add users to accounts) and deprovision users (remove users from accounts) into HubSpot programmatically. You can read the entire documentation of the API in our docs on this page - https://developers.hubspot.com/docs/api/settings/user-provisioning - with a password Hub$potUser!
We're still adding more features to the API but let me know if you have any preliminary feedback.