How far along are you?
The answer is relatively simple: "Go back to the source."
If you cannot complete it by yourself, go back to where you found the first set of people to help you. Problem: you better have a good presentation, and a good design document - people want to know what they are getting into. If you can't explain the project to someone else, let it go for now.
Even the big guys and their commercial products run into "turn over." The real trick to managing it affectively, is to let the previous members of your team know you are moving forward AND when they are ready to rejoin they are welcome.
By the way, the best way to get backmotivation is to never let it go. Find out what your members find motivating and rewarding - DO IT. Be sure to find a means to recognize your team for their accomplishments - past and present. Websites can be great things for both - plus it will let your team know about the game's progress.
If it hasn't hit you yet, the above is about MANAGEMENT. Unfortunately that is the evil lurking in the heart of any group project. If no one does it, there is no project.