I have been invited to a conference in USA. I am going to apply for a B1 visa. Unfortunately the invitation letter doesn't have a signature on it. Only the email id and the website is there. Would the letter be valid in this case?
The right answer to this is that it depends on the consulate officer.
I got my B1/B2 with only a single document, my employment/income proof. From what I can understand, as long as consulate officer is convinced that you have enough money to sponsor the trip and enough reason to come back to your country, it is enough for them. Strictly speaking while an invitation letter is a plus, it is not mandatory. It just helps in increasing your chances, in case consulate officer gets suspicious that you are up to no good.
If you will need to submit it to the consulate you will need to have an actual letter signed by an actual person.
If the letter is unsigned it could be generated and printed anywhere and not necessarily be from the person or an organization that is inviting you to the USA. My friend had to get a letter by mail (FedEx, UPS, DHL will work just as well).