Do we need a per app License to share a Model Driven App in a Sand Box environment ?

Do we need a per app License to share a Model Driven App in a Sand Box environment ?
Depends I guess. If the user/s already have a Per User Plan then No, if not then Yes. However, I don't think you can publish the app in any Environment, other than the Default Env, without a Premium licence. At least this is my understanding.