I'll do some work on the vocals (I've not yet heard the backing track... I'm downloading it as i type this).
Why dont we let the whoe thing be spontanous, just let who wants to do what ever do whatever... I mean for example there was a dance record that came out last year that had a very cool vocal that "echoed" the guitar part... and things like that would have to be done as we go along (if i was to do them).
Am i allowed to put some guitar on there?