It kind of depends on what everyone's plan is for when you meet up. If you are meeting to go and eat, then plan to meet at that food location. If you are planning to meet to leave then plan to meet near the exit.
Now, if you are planning to meet up just to have check-ins from time to time, then pick a place that might not be too crowded. Do not plan to meet near the main icons for the parks as these get crowded as people stop to take pictures.
The PP had a good idea for picking places with seats, but some of those are going to be out of the way or busy unless it is outside of the main eating times. Also do not make plans to meet during a parade/fireworks time unless it is at least 30 minutes before or after as it can be hard to get through crowds at that time.
My suggestions:
MK: Liberty Square Market (beside Hall of Presidents and across from Haunted Mansion); it is an open area that would make it easy to find each other, plus it is easy to get to from all areas, except maybe Tomorrowland
Epcot Future World: Pick a side of the fountains, such as the side facing Mousegears or the side facing World Showcase. This area can be crowded, but is centrally located.
Epcot World Showcase: Pick a country and maybe a landmark. Example: Japan at in front of the White Heron Castle, tall fortress structure, on the side facing the shops of Japan.
DHS: Dinosaur Gertie's or Min and Bill's Dockside on either side of Echo Lake
AK: This one is a bit more difficult with how it is spreadout. It might be best to pick outside one of the restaurants near the Tree of Life or to pick a specific attraction.