The only thing I've found that comes anywhere close to this is Adderall, and that has awful side effects.
I've been in several of these type of groups over the years, and they usually fail because people flake and no one calls them on it. I think we've finally gotten the formula right though. The main thing is that people who aren't doing their part will get a warning and get removed from the group if they don't take it to heart. We make it indirect....instead of calling someone on it directly, you tell someone else, and they address it with the person.
I could describe our whole system in more detail (there are a lot of little tweaks we've made), but the main thing that is important is that you have to think of it as a complex engine that needs fine tuning to keep "firing on all cylinders". Don't be shy about telling others what you need. But mostly, make sure that everyone is vigilant. If you flake on the tasks you are committing to, we can work with that. If you flake on monitoring others, you get a warning and if you keep doing it you are out.
One of the things we've found works well is to structure tasks so that getting started in the morning (or after a long break) is easy and -- if possible -- fun. That is, you never leave a task in a state where you have a long "warm up" period. Get it to a place where it is satisfying to work with before you put it down. This often means finishing up a major task, then starting a new one and just getting it going before you call it a day. And of course write some documentation (which could just be a todo list) before you stop work.
We're eventually planning on doing this in a bigger way (with a web app, etc), but for now we're open to bringing new people in if they are very VERY committed. Get in touch if you are interested (rjbrown at gmail). You need to be willing to use Skype video chat several times a day.