We unfortunately don’t have a way to automatically add users to an organisation yet. You will have to manually invite them today. You can however disable users from signing up following this guide.
@latic just to be clear they don’t need to have their emails on github, if you ask them for their emails, you can invite them, and then they can sign up with their GitHub id from the invite email they receive. If you have no way of getting their emails then marking them in robo3t seems like the best option. We have a feature request for this here
Could you leave a bit of information on why you can only invite them via their GIthub Ids and not get access to their emails? It will help us understand the usecase and prioritise the feature sooner