You can lan plenty of people on the same copy, the limitation comes into play after the party is over.
If player 2 and/or 3/4/5/6 (assuming you are player 1) want to keep the game, update or play online after the lan. Then you have to shell out for the serial.
When playing lan, all you need is to have everybody on V1.05 and it should work just fine. One copy would be all you need in this instance. It wont even request the serial 