First of all you'll need at least 2 Xbox or PS3 controllers connected to your computer.
Move with left axis, shoot and aim with right axis.
The prototype is one level with capture the flag mode, you need to get other player's flags and take them to your base. You grab a flag by collisioning with it.
If your flag has been taken by a player, you can't score, you need to kill that player or wait for him to score.
The red arrow around you always points to your base, so that you know the way home.
Beware of the glitches.