I'd say it's probably pretty difficult. I'm certainly not an expert, but to come in to the US on an Investment Visa, I think you need a few million in the bank..
To do it as a Employee, they'd need to do H1B (which I don't think applies here), or standard immigration.. If it's a standard work-visa, there's a problem in that you're not working for anyone..
It probably isn't impossible, but it'd make things substantially more difficult.
To get a H1B to do Y Combinator is probably not an option at all. H1B require certification of the employer etc, which you are unlikely to get for a brand new startup.
If you find a way to be in the country then you can probably worry about VISA when you know your startup is going to be a real company and you'll take investments etc.
To do it as a Employee, they'd need to do H1B (which I don't think applies here), or standard immigration.. If it's a standard work-visa, there's a problem in that you're not working for anyone..
It probably isn't impossible, but it'd make things substantially more difficult.