I would recommend, at the very least, trying to contribute to an open source project first, so you can get a sense of how to collaborate with other developers (You don't really mention if you have professional or just personal experience, so i'm assuming the latter). It's actually a very difficult thing, to give up control of your project and place it part ways in the hands of others, and so you might want to experience being on the "Community" side of that fence first.
Just my 2 cents on the issue.
As for how to protect your IP in an open source project, I'm not entirely sure, but I would imagine the right licensing scheme would go a long way, but I'll leave speculation on that to the folks who are more familiar with the ins and outs of the various flavors of open source licensing that exist.
Otherwise, good luck on your game!