They actually do ask. For example Pre-September 2008 if you were in a park wih evening EMH then at a certain time the CM's started filtering guests out and checking resort Key to the World (KTTW) cards to see if you were a resort guest. If you were then u got a wristband if not then you made your way to the exit. Now they have done away with the wristbands and they check at the attractions, asking for your KTTW and/or ID. Different people are reporting different experiences, some strict CMs and some CMs that don't even check. So I think you should only depend on EMH if you are a resort guest. With the intermittent enforcement you might be able to just slip by, but I wouldn't guarantee it. So you are correct if only 4 people are doing something then you only need 4 keys but if everyone wants to participate then they might not be able to.