Upbeat Perform Music App
Online tool where you can record or upload a backing track, and then other people can record to it online at their own convenience. Works in a desktop browser, nothing to download or configure.
After everybody submitted their parts, the project owner makes adjustments and mixes the recordings together to create one final collaboration video. Can even exclude the backing track at the end for a "live" feel. Post the final video to youtube/dropbox/...
Free for <4 people; when there are >4 people it just puts a watermarmark over the video, so it's still usable for fun.
Upbeat Perform Tutorial:
(jump to 13:40 for backing track explanation)
Bring Me Sunshine: Using Upbeat Perform:
Hope it helps