If you need lots of early contributors, then you're not really in the position to do a startup. It's a lot of work, so I get why you're tempted to involve a lot of people, but it never works out well. Decisions have to be made and the way you handle it isn't going to be appreciated by everyone. This can, alone, kill your fledgling enterprise.
You can't solve this with contracts. The answer is to do more of the work yourselves and keep the team small. Involve others when you have something real to involve them in.