This was a real scenario I faced and the company/founder was a complete stranger to me- I forget if he just found me on LinkedIn or it was through angel list- I would not have been a cofounder just their CTO. Either way I ended up taking another offer with somewhat lower risk- they were extremely successful but turnover there was on the order of 25% per year. And I did end up taking some people with me and lasted about 5 years there so it all worked out.

I check in to see if the other company ever made it anywhere and nothing ever comes up so I guess I wasn't missing out on startup riches.