If you wait on the sidewalk where that parking lot begins after the game, you'll see fans there waiting for the players.
The main rotunda entrance at Citi Field is near home plate -- this area players come out of is to the right of that entrance. After the game, walk out of the main entrance, go left -- you'll see fans waiting there for the players.
The picture I attached: The area circled in purple is the main entrance of the stadium where most fans enter/exit. The red area I circled is where the players enter/exit and where fans wait for them postgame.