You're not liable for their usage.
However, you will be held responsible for the content they are downloading.
Your ISP will give you an IP address for your router. The router will automatically assign IP addresses for each of the computers connected to it through DCHP.
The only address that will be seen is the Router's IP address and that will be traced to you.
Unless you can get some kind of software that tracks downloaded files and where they finally end up but I bet that's expensive.
You may be debating with yourself that the risk is perhaps small enough to let it go.
But if one of those guys downloads kiddie porn, you'll be sunk... not the guy who downloaded it.
Your ISP will tell the cops your IP address, what was downloaded and when. They'll turn over your house address and the boys in blue will beat a path to your door.
You see, unless you have a server set-up that keeps track, the router will never remember who downloaded what and from where. Once the information hits your router, there's no record of where that data is going to end up within the house. unless, as I pointed out, you have some sophisticated software running.
The police will not be able to get a court order for each computer that connected because if only one person is guilty, then the other five will be innocent and that would violate their human rights.
But you take the fall because you allowed them to use your internet.
I would suggest that the only way to really do this is to buy a wireless router and a wireless card and secure the h*ll out of it. They will know there is a wireless box somewhere but if you enable the SSID, they will not be able to access it and they'll not know it's you - it could be next door.
Is this rented accommodation or do you own the house as well?
Put it this way, I wouldn't take the risk.