People always say to avoid the busy areas...of course that is often still where we choose to meet up just because it's easy. I think pretty much a good place is a place everyone can get to easily. For example, a good place at MK I would think would be Liberty Square...you could choose a spot like at the tree, or at the bell, or in front of Hall of Presidents. In MGM I would probably say something like Gertie's because it's out of the way. Epcot, would depend on what the itinerary for the day was. There may need to be a country to meet up at, or a section in Future World. You could do something fun like meet at Ice Station Cool! If you wanted a spot in WS, I would choose something off the main path, like the fountain in Italy, the Viking Ship, or the Stave Church. AK is harder because everything feels so surrounded. We don't do alot here, so we have never had to meet up with anyone....
Anyway, just some ideas.