They would need work visas and (assuming that they don't qualify for a country-specific visa) the potential options are the H-1B and O-1 (and possibly the E-2 depending on their countries of citizenship and other factors). Because this analysis is so fact-specific, you will need to have a consultation with an attorney to explore the options.
Quick follow-up: In principle, would it be possible for them to keep their current job while working on the startup, or would they need to leave it entirely?
