If you are fully convinced yourself that SFI is legit, then first expressing what convinced you is the place to start. After giving your justification if there remains any doubt for your prospect, invite them to join SFI for FREE and investigate the business and job opportunity for themselves. They can "try" it out for as long as they choose and the only cost to them is a bit of their time. After that, if SFI is not for them so be it. Marketing isn't for everyone as we all know.