Zombie mode: Team effect?

When I join a zombie mode room, I always see "Team Effect: 0 members", which is above how many revive tokens you have. I have NEVER understood what that was about. But what is it? And why is it always at 0 members? I've had some people tell me that it has something to do with the slaughter tickets. And well, I have slaughter tickets, but the team effect has never gone beyond 0 members.

http://imageshack.us/photo/my-images/20/ljp.png/

if i remember correctly, the "team effect" thingy works like this: have items from the boss (all i think) and if u all have items then u get x% exp/gp + ... cprrect me if i am wrong

i think u must play with clan Mates