Can you give me some more info on what specifically you're looking to do/build? HubSpot doesn't surface any user info since the OAuth flow is intended to be portal-specific. It sounds like you might be depending on user info for some sort of user-specific connection with HubSpot?
You'd probably be better off using user_id since that's directly related to the HubSpot user that completed the authentication process. user_id is globally unique; keep in mind though that there is not a 'Users API', and there's no way to push/pull user info to/from HubSpot.
Thank you for your reply! We will use the user_id for matching the HubSpot user with our application-internal user object.
Since there is no 'Users API', how can I get the user's full name? Is the email address safe (at least for a short amount of time) to match the response from /oauth/v1/access-tokens/:token to the remaining information from /owners/v2/owners/?
Is it possible to filter the response from /owners/v2/owners/ by anything else the the ownerId?