business partner offer, need help

Hi there! I have questions about a business partnership we were offered and I am so confused.. My father in law wants us (husband and I) to become 40% partners in his business. It is a very small freight (trucking) company. My husband is an employee right now. He is paid an hourly wage. We're not exactly sure why his father wants us to become partners. Is there tax benefits? Who does it effect? How does loss and gain for the company effect us if we were 40% owners? What things should we be aware of? I'd also like to add that we are not investing any money or anything to become partial owner. So basically how would this benefit us and/or him? How could this hurt us? How would this change our taxes? We normally get a refund, would that be gone?

Thanks for any help with this! We just don't know what to do..