Short answer
HubSpot's KB: SCIM creates and manages HubSpot users automatically from your identity provider. Okta and Google have guided setups; providers like Microsoft Entra ID use a custom SCIM integration. To assign seats, purchase them in HubSpot first, create a permission set with a paid seat, then set the user's custom attribute in Google.
1. In HubSpot
The relevant screen in my demo portal.
2. Worth knowing
Follow Google's own auto-provisioning instructions for the Google side.
3. Related
Users and Super Admins and login sessions.