Ah... The teams are arranged in no particular way. You all sort yourselves out to red or blue by standing either side of the courtyard. It's not as haphazard as it sounds, because mates/teams stay together, and everyone else gets split down the middle, regardless of numbers.
They then do a count and balance the teams. Any large teams are asked to make sure they apposed another, so you don't end up with 4 teams vs a group of random "walk on" players
I believe rift prefer you to book your day, by making a post on their forum, so they can arrange things properly. But it's free. That said... People often turn up unannounced.
They also have feedback and loads of photos on every day they run, so you can get a feel for what people think, before going. It's all on their page/forum. Last game they supplied a machine gun emplacement, with flash generator and sound effects... It was pretty badass, by the look of it...