The Fab Five are Mickey, Minnie, Donald, Goofy and Pluto.
To high-five them first approach them and wave
to interact and then
to high five them.
Very easy to do as you have to interact with most of these characters (with the exception of Pluto) in the first few missions.
At the start of the game all will be on Main Street but if you are a bit further along you can usually find them in ToonTown.