Your manager’s authority. By definition they get to say what is part of the contract, on behalf of your coworkers. They can still complain after the fact, but

Your manager’s authority. By definition they get to say what is part of the contract, on behalf of your coworkers. They can still complain after the fact, but will need to convince your manager(s) to reverse the decision. If your manager doesn’t have that authority you need to deal with HR or whoever had it, but by definition there will be someone holding that role in your company. Of course consulting coworkers on important arrangements makes thing go smoother, but it’s only consulting, the chain of authority doesn’t disappear.