Google docs may be your friend here. You can add a bunch of contributors, and maintain a read only copy for the final edit. Users who don't want to add anything creative, might well enjoy correcting spelling and gamma. I know a few users on this site who get annoyed at peoples bad spelling, perhaps you can let them at it
Also I recommend dividing the project into chapters, working on a book or story "as a whole" is daunting, plus your mind races to the end, middle and beginning all at once. Chapters are easy however, when focusing on 6000 - 10000 words at a time you're able to place yourself in the moment, rather than in the future.
Writing is like programming. You plan it, or you live it. Sometimes both, bullet pointing your tasks per chapter is a good idea too. When I have trouble writing and focusing, I bullet point instead.
* Opening scene, character in in pain, laying on the ground.
* Asks himself questions, why is he here, what happened, etc.
* Atmospheric description, the smell of smoke, the roughness of the tar mac, etc.
* Brief Flash back, conversation with lover, before sudden loud noise.
* Event! Forced from memory, main character gets up hurriedly as a car, it's horn honking barrels towards him.
* Horror of recognition
* End chapter, his lover is dead at the car's wheel. What happened to her and why can't he remember!
Once you're in the mood you can convert something like that into a few thousand words easily, and you've got yourself a chapter!
Repeat this 12 - 30 times and you'll have a novel. Writing is just like programming, one step at a time.
P.S. I practically live on google docs these days