If you host a party and serve alcohol you could be held responsible in the event of an accident. The biggest thing that you should be aware of is to show caution about the alcohol intake of guests. There have been situations where social hosts have been held responsible for the behavior of their guests.
You should always serve food with the alcohol, and keep an eye on how much drivers are drinking. Some people have their guest place keys in a bowl or basket, and before the drivers can leave the host makes a determination on the state of the guests.
If your guest appears inebriated or impaired when they prepare to leave the party you should ask them to either stay, take a cab home or have a sober driver drive them home. If you host a party and you clearly know the guest is impaired but the guest insists on leaving, as hard as it is you should call 911 and report the driver, you may be saving lives.