We find that gaming for LAN parties is always a pain, I've said this before but I think that the ability for one steam user to create a group of steam ID's (your friends) and then use a credit card to rent a game for X many days for that group, obviously the price would be based on the length of rental and the number of people renting.
Then the friends of the group simply get a msg on steam inviting them to participate in the group, when they accept the game downloads and installs, you could even have it download on 1 PC and then have the steam client act as a distribution node so that from that 1 PC its distributed across the network to the other clients assuming they sit on the same subnet at the time.
Once the time expires it simply drops off your list of installed games like other products on steam do.
You could have options for distributing the cost between the members as well so that each individual can have the option to accept/refuse their share.
It's an amazing deal all round, gamers get an additional value added service, developers/publishers get a rental service that they make money on where they traditionally would not, steam get their cut and sales of good games actually increase based on getting to demo the full product.