As soon as you have an EAD you can start working for it. Anyone can start a business, you just can't work for it until you have work authorization from the INS. That would be your green card, and EAD, or an H1-B specifically for the business you've started.
You may be asking yourself "how can I start a business and not work for it?". You invest the money to get the business going, but hire staff to run it. You are really just an investor in the company at this point.
Been there, done it, got the t-shirt.
Rav