I apologize- I thought volunteers were contributing to your
private repos. If I understand correctly, your issue is that you have (say) 10 employees accessing private repos and 100 contributors to public/Free ones, but you're to be charged for all users you add to your org? I can see how this would be frustrating.
I see two possible solutions that don't force you to switch vendors:
1. Have non-employees fork & submit pull requests.
2. Split your private stuff off to a different org & formally separate free stuff from proprietary, make the free stuff community managed.
If these are problems, I'd maintain that this is a "have your cake and eat it to" problem, on the one hand keeping ownership & control of the project and reaping the attendant benefit to the edx brand, and on the other hand getting people to hack on your stuff for free. But in any event this is a broader existential issue that exists across the OSS world right now (see express.js), so I'm probably reading too much into your case. :)
