| ...or, "where were we meeting again?"
You've got this great idea for a gathering at a great place, and lots of great people are going to be there, and you're all going to have a great time! You're all set, right?
Wrong. You're still only halfway there.
Just telling people when and where to meet is only the beginning. You still need to arrange to Locate, Coordinate, and Communicate.
Locate. Where will you be initially? Where will you be later? People are going to be late -- some through slackassedness, some through getting caught in a Kafkaesque nightmare of missed connections, and some because of bona fide emergency situations or otherwise unforeseeable or unavoidable delays. There are few feelings worse than getting to the meeting place late, knowing your friends are here... somewhere... but not knowing where, or how to find them.
As part of Locate, it's good to have a way to Identify. Something distinctive one or more members of the group can wear, or something about them. Something like a fluorescent yellow t-shirt, or neon green hair, that will make them stand out from the mass of humanity.
Coordinate. How will you determine what deviations, if any, will be made from your Location plans? You thought the steak place would be great for lunch, and then you find out three-quarters of the folks with you are vegetarians. Oops. But this doesn't have to be a disaster if the change is carried out in an organized fashion. Will you split into two groups? Will the carnivores suck it up and partake of the salad bar with the leaf-eaters? Will there be somebody to stay behind and watch for stragglers at the original destination? There should be an agreed-upon (or at least, consistently-implemented-by-force-of-arms) method for deciding such things, if for no other reason than to cut down the whining.
Communicate. If you exercise your Coordinating options to deviate from the plan, how will you get word back? In some cases this is simple -- most hotels, shopping malls, etc. will be happy to take messages at the check-in/information desk. Other times you may need to post a sentry at the initial gathering spot, or just agree that latecomers should wait there at specified times -- say from five minutes till to five minutes after the hour. This last method has the double advantage of setting definite expectations while allowing everyone relative freedom of movement and some degree of freedom in arrival for latecomers -- like public transportation, if you miss one opportunity, you just kill time till the next one.
It's usually not necessary to implement all three suggestions, but it doesn't hurt. Because hey, people forget things. Ideally you have the stragglers seeking you and you looking out for them at the same time. As with magnets, two attractive forces are better than one. | 
