Tu/Lei are singular; Voi/Loro are plural. Another point, from my teacher in Florence: "formal" does not equal "honorific". You use "Lei" or "tu" based on how well you know the person, or how familiar/close you both are. Eg. if you used "Lei" when speaking to your friend, you have placed a distance between you both. While "tu" is used commonly, if you use it in the wrong context (eg. business meeting), you would come across as far too friendly and a even little disrespectful.