Recently Sonic Pi 2.11 has been released (so far for Windows and Mac). With the addition of a cheap audio usb card (eg SABRENT card £6 from Amazon) and a cheap microphone (you can use one off a skype type headset) you can record a 4 part rendition of Frere Jaques being sung, using only one person.
I have put together a video with full details which you can see here https://youtu.be/LmD9rO-KJ_0