How to do this: answering a few questions causes usergroup promotion?

[Alfa1]

Does anyone know if there is a modification (free or commercial) that allows me to add the following function:

Answering a number of quiz questions gives additional usergroup promotion.

I want to restrict a few subforum to members that have correctly answered a few questions.
I assume the best approach is to use an additional usergroup to give access permissions. And to somehow let a correctly answered quiz/questions cause promotion to this usergroup.

Does anyone have an idea how to do this?