Please remember that you can register someone else to the vaccination booking system only if that person gives consent to that. Moreover, under no circumstances are you allowed to impersonate someone else in order, for instance - no later than upon entering the vaccination point this trick will get rumbled and you will not get vaccinated at all.
If you - in good faith and with that person's consent - decide to register someone who needs that (e.g. an elderly being not so keen in getting along with a computer), it is surely a good deed and we even aim to make that possible. The process is the same as if you would register and book a timeslot for you yourself, only the personal data have to be replaced by those of the person you book the vaccination timeslot for. Watch the telephone number you enter - the notification messages and login data can be received only at that number. Therefore, please consider if you enter the person's number or your own one. Since it is the health insurance number that acts as your unique identifier, it is possible to carry out multiple registrations under one telephone number.