(I am not a lawyer, but probably you aren't either, so at least we are on a level playing field ;P.)
> From your mother-in-law to the UPS delivery guy, when someone else sets foot on your property, you could be liable if they’re injured, whether you did anything to cause the injury or not.
http://budgeting.thenest.com/can-sued-somebody-getting-hurt-...
Due to the US notion of "join and several liability", one would imagine in such cases that both the renter and the landlord would be sued by the guest, and in the end it would likely be the landlord (who probably moves larger amounts of money) that would end up paying, requiring them to get it back from the renter.
I imagine this is one of the reasons (in addition to situations involving squatters, etc. <-) why many rental agreements actually have limits on the ability for you to even have guests that stay for more than a few days at a time more often than every few weeks or months.