My parents had visa interview couple of months ago. I sent them a invitiation letter. They took their passports and a invitation letter to the visa inteview.Visa consular didnt even read the invitation letter just asked them few questions about me. They said interview didn't last more than 5 mins.Since I'm a US citizen it was really easy for them to get visa.If you are a US citizen or a GC holder it shouldnt be that hard to get a visa for your parents.