Can an app be designed where some users can get it for free (designated by the maker) and others have to pay for it?

I am looking to design an app which can be used on an iPad that I can allow members of my mentoring program to use for free but I want to charge the general public. So I need to be able to pick the users who get it for free

Is this possible? Or is it a free or cost thing?

your help is much appreciated

Regards

Bec