Is there a reason you are importing the users instead of just creating them with the POST /api/v2/users call? I would think that this second call is closer to what you are truly intending. Unless you need to import a large set of users at a time, I would recommend just calling the create users endpoint instead. This will reply with the new user object which will have the user_id in it and you can then ship the email after you create the user.
The job results won’t return the list of user IDs. If you have a large list of users you will have to load the users and then find them a different way to get their user ID, such as searching for them by email. Since this would require the same number of Management API calls, it is probably simpler to just use the create user call to begin with.
If you are just doing this bulk import at the beginning of your use of Auth0, then you can just bulk upload and then get all users and then send emails to everyone individually after that. Then you can use the management API for creating users in the future.
Here is the create user doc: https://auth0.com/docs/api/management/v2#!/Users/post_users
Make sure you catch 429 errors and retry again after pausing your operation. Especially in a dev environment or free account you only get 2 calls per second (with bursts up to 10). So this will be important for making your uploads smooth.