Next year for our high school reunion, my wife and I are considering the possibility of doing an apocalyptic-themed survivalist LARP at a camp in the middle of nowhere Colorado. This is something that all attendees would be interested in. Some of them have no RP experience whatsoever, some of them have RP experience but no LARP experience, and some of them may be experienced. What set ups would work with our group size (20-50) and the mixed level of experience? It doesn't have to be a formal system; it can even be something that you made up on the spot, but I don't have a lot of experience organizing LARP so I'm looking for advice.
One idea that may fit your theme would be Humans Vs Zombies. It has a simple learning curve and is geared for mid sized groups like this.
One thing to consider with HvZ is each player is going to need either a Nerf gun or pairs of socks that they are willing to damage/lose. The game can be run without Nerf Guns with very little change.
Short and simple: The game revolves around group of Human Survivors trying to survive in a location. This becomes a challenge when Zombies start to overrun the surrounding areas. The game starts as Zombies start to attack the group(1 - 5 Zombies at start).
Zombies can turn Humans into more Zombies, Guns and Socks/Marshmallows can stun Zombies, and the aim is to survive a period of time. Humans win if they last till the end, Zombies win by converting all Humans. Zombies can also die by starving.
The rules and concepts of the game are easily flexible to accommodate level and length of game play. Some games can be resolved in a day or two, while others last for weeks.
More on the game can be found at http://humansvszombies.org/
Also missions and other RP elements can be added to enhance the game.
If you want you could limit how much the guests know before arrival(just that the LARP in question involves Nerf guns and the rest will be explained).
Then hold a meeting when they have all arrived. In this meeting start with a back story on how Zombies have overrun civilization, they are a group of survivors. During this have the seed Zombies brush close to create atmosphere. Following this have the leaders explain what is known about the Zombies and start the game.
To add to RMDan's post, you can mix it up with a scavenger hunt as well. We played a game called 'Tribes' where there were four teams (red, blue, green, yellow) each with a base, a flag, team members... and a laundry list of BS they had to go get. There was no real attacking as no one had weapons, but the point of the scavenger hunt was to collect items for the tribes to use in order to capture the flag, in which they would get that tribe's items... as well as their flag. Some of the items were rope (to lasso an enemy), a lighter (for light), a key (to enter someone else's base), a map (obvious), a pen, water bottles, food, shield, foam floaty tubes, etc. It was like survivor, except without the voting and the 'me!me!me!' aspect. It lasted about two days and was a lot of fun. It was like trying to play paintball in a national forest without the paintball guns, and having to use your brains. Terrain permitting, this could be quite fun for large groups, as I did this with about 20 people.