There are 2 main ways to excite people to work for you unpaid:
Number 1: Help people get their foot in the door.
If someone works for you for free, they might just want to enter your industry but lack the talent & connections to do so.
Make it clear to them that you are their door!
Number 2: Help people build their resumes/portfolios.
Unpaid workers usually have a chicken & egg problem.
They need experience to get a job but cannot get a job because they lack the experience.
Make it clear to them that you will go out of your way to help them build their resume/portfolio.
Make it clear to them that you can help them get a well paying job in the near future.