The best advice is "agree to terms before you begin, and don't worry about it".
Ideally, you'd use really generous terms as I suggest in my answer to the question:, which will further help to reduce tensions as the prototype/idea is fleshed out.
Failing that, you have the ultimate in "Chestnut Advice" about startups: If someone (or the whole team) pushes you out of your startup, you really should take it as a sign that the team wasn't going to work anyway…
Simply possessing a percentage (even controlling) in a stock doesn't prevent people from leaving/stealing your idea, realizing they are faster/cheaper/better in some way without you aboard.
Perhaps the best advice in these situations, is to pick yourself up, dust yourself off, and focus on always being valuable to your team, rather than worrying about whether you can can somehow keep control over them.