I know many people have posted videos showing you how to get two people in the lighthouse glitch (and many people are having trouble with them).
Well after playing around with a friend we came up with a excellent way of having two or more players using the glitch to help them to high rounds... unfortunately I do not have a recording device so here is a step by step:
Step 1: Open any door to the light house apart from the one next to the AK74u (we use the door near PHD Flopper as you do not have to open the stairs) Step 2: Open the door that is in the room where the old characters are trapped to get to the bottom of the lighthouse Step 3: Have one player get into the glitch next to the AK74u Step 4: Have the other player go into the bottom of the lighthouse and stand at the window to the left (the one to the left as you first come down the stairs).
It's as simple as that!
The only zombies that the players have to worry about is the ones at the window, as all the rest will go straight to the door.
To do this with more than two people simply have everyone stand as close to the window as possible.