I know FreeToPlay games are financed by advertisements and/or selling additional content, but what about an Open Source single player game to which several people have contributed? And how can a fair sharing be determined?
I would suggest that you sell:
Distributions of the source: Build versions of the source code, tools, etc that people can work with. If they are modifying the source themselves then this won't help them, but if they just want to include a dll for example, this can simply things especially if your build process is complex. Another options is to sell incremental revisions of the source and open source previous versions.
Art, Basic Levels, etc: As mentioned above, providing art resources and building block maps, objects, and other assets simplify development for others.
Server Rental: If your game is an open source multiplayer game, you can rent out time on servers you run. This makes supporting the game easier for people building off of your game, and ensures stability (this relies on you of course) for multiplayer games.
Advertising: A no brainer, advertising on your site and support forums could earn you something
Support: While I generally view selling support as evil, perhaps selling room in online training sessions, access to advanced tutorials, or "1 on 1"-ish support wouldn't be so bad (likely more like 1-or-2-on-team but hopefully you get the idea).
Microtransactions: You could sell content in game, or only provide certain content in the "premium" version of the game that YOU built. People building off of the game would have to either go without said content or pay you to include it.
Commercial Licences: While your game could be open sourced for non-commerical purposes, you could require that you be paid a certain amount or a cut of the profits for people planning to sell derivatives of your work.
Promotional Items: If your game gets popular enough, you could sell T-Shirts, etc relating to the original game.
Sponsorship Period: Following along the lines of point 1, you could sell the game for a certain amount of time and THEN open source it.
Donations: Simple--ask for donations to your project (this would probably look bad if you were doing too many of the other things but it may be worth a shot).
I'm sure there are more ways I didn't even think of as well. As for sharing, that's really up to your team and you but as this is an open source project and not something built in a corporate environment it would presumably be split evenly amongst you.
I would start searching the blogsphere to find people who have. Steve Streeting who lead the Ogre project was quite open about his experiences about making money from his open source engine 1.
Some projects have released older source to stimulate interest and modding after an initial closed-source start. The people at Wolfire 2 released some source. Introversion have been recently selling code for Multiwinia 3.
Joining an interest group at IGDA 4 or BCS 5 could also be a pro-active step to finding out more, and larger organisations like that may have resources to answer your legal questions about revenue sharing etc.
You could also have people donate/pay for you to work on new features once the game has a following. For instance, if a group of users really want the ability to ride horses in your game, members would donate money to have you put that at the top of the priority list.
It's certainly possible to make money with it. At the very least you can ask for donations or distribute a version with advertisements. It will be difficult, and there's no guarantee you can make the same amount of money, but it's possible.
If all the contributors agree, you can always close the source, improve it, and then sell that. That's probably a much better approach.
I'd try to split "fair sharing" based on how much time the various contributors spent on it. It's quite possible nobody involved will be happy with the results and there will be no agreement that can be made. On the other hand, if it's open-source, they also can't stop you from making money off it - selling open-source software isn't illegal as long as you leave it open. If you can come up with a method to make it profitable, then you'd deserve the money earned from it.
Without knowing more about the situation and the game, there's not much more we can say.
The way people like Red Hat make money from open source is by providing support contracts. It's obviously not practical to charge customers like this, but if someone wants to base a game off your code, why not set up a formal support contract?
For more traditional revenue, why not open-source the game code, but charge for content packs and art? If people want to improve your code, or provide their own content, great, but your's is going to be the best, most fun and most professional, natch.
To understand what you can sell you first need to understand what your game consist of and it breaks down to the game engine, client/server engine, art assets, rule/value/balance assets.
So if you want to open source the game and still sell something you have to chose from the above one or more to keep.
Additionally you can also sell the "service" whether it is access in multiplayer games or in game items and so on.
You can also go the donation way based on the promise that you will keep evolving the game.