There are a couple of steps you can take, but neither of them will give you the protection you need.
First, as Felgall suggests, you can get your collaborators to sign a non-disclosure agreement. That will prevent from telling the world - or their friends - about your idea. Going further, you can get them to sign non-compete agreements, with the aim of preventing them from putting your idea into practice in competition with yourself.
But neither of those steps will really solve your problem. The basic truth is that you can't legally protect an idea. You can only protect the implementation of the idea. If you have an idea for a website, you have to actually develop the site before you can claim any rights in it. If someone else develops it first, they will enjoy full rights to the version they create, and there's nothing you can do about that.
However, the situation is not quite as bad as all that. Contrary to what most people assume, stealing an idea is not a particularly useful or profitable thing to do. It's not the idea that has the value; what's important is the skill, effort and dedication needed to put the idea into practice. Even if news of your idea leaked out, it doesn't mean that there is a body of skilled developers out there who will grab hold of it and build something useful out of it. The chances are that you will still be the best person to do that.