The Sphere Grid becomes accessible after Rikku explains
how to use it in a short tutorial. Each character who joins the
party has a pre-designated starting point on the grid. While each
of the characters has his or her own section, you can remove the
locks between the different sections by using the appropriate key
sphere.
Thus, character advancement is limitless. While in the Sphere Grid,
press the Select button to zoom out from the current character's position
to see his or her relation the other characters. Press
and to change the viewing angle
of the grid you desire.