It depends. Are you talking about everyone arriving at the park at around the same time - or meeting to leave? If so:
MK: Town Hall end of Main Street.
EP: In front of SE.
MGM: By Sid Cahuenga's (sp) shop to the left after you enter.
AK: Don't know.
Now, if everyone's been in the park for hours, but you're just re-grouping:
MK: The Fantasyland side of the castle
EP: By what used to be the Odyssey restaurant between WS and FW.
MGM: Great Movie Ride plaza.
AK: Don't know.
HTH.