Help the main cast by for example exchanging ideas and give them pointers, advice and criticism throughout the competition. They can ask you for help or let you hear their entry before they send it to the judges. In other words, do what a mentor does. :) If there's more than one mentor, the contestants and mentors will get a chance to choose each other when the main cast is revealed. You will be credited in the episodes, and maybe get a chance to do an interview once, telling about your experience with them. Other than that, there will not be much participation IN the episodes, it's mostly behind the scenes.

For this audition, please present yourself and tell me why you think you would be a good mentor. What are your qualifications? And don't be afraid to show your personality! :) Also, I will probably be much more confident about your abilities if you audition as a judge as well, since a good mentor is also a good judge! If you want to show that ability but are not interested in being a judge, you can just audition for judge and tell me you're doing it for the mentor part!