See one of the comments:
"I have a lot of experience with this, having done it several times myself. Proving the company is 'real' to the USCIS can be tricky, because they don't consider companies with cash but no revenue to be viable. Luckily it never usually gets to that stage of analysis for a simple H1B (although green cards are a whole different matter). Just make sure you have an office address,letterhead, phone numbers, a bank account and good lawyers. The USCIS will often pick up on little details that show the business is being run out of someones backyard.
The hardest part is in the early stages, prior to transferring the H1B visa, because technically the visa holder can have no involvement in the company i.e. cannot attend meetings or talk about the opportunity without violating their status.
One approach I used successfully was to get my accountant to create an LLC. The LLC held my shares in the new startup. My immigration attorney then applied for a part-time concurrent H1B to work for that LLC as a board member a couple of hours a week, in parallel with myexisting H1B visa, allowing me to be involved from day one (you can start work the moment the visa is filed). When the startup was funded, they then filed a H1B transfer for me. "